President Donald Trump said Wednesday that he has directed the Navy to “shoot down and destroy” Iranian gunboats that “harass” U.S. ships, and U.S. officials said the threat was meant to warn Iran not to repeat what the Pentagon described as a provocative encounter last week in the Persian Gulf.

It was the president’s most direct threat of military action against Iran since the two nations came close to war in January, when Trump ordered the killing of a top Iranian general and Iran retaliated with attacks that harmed U.S. forces.

Although Trump said he had “instructed” the Navy on the U.S. response, the nation’s No. 2 military officer later said the decision to use deadly force against an Iranian ship is up to individual Navy commanders, depending on the level of threat to their ships and sailors.

Trump’s directive, issued in a morning tweet, came a week after the U.S. military said 11 small vessels belonging to Iran’s Islamic Revolutionary Guard Corps conducted “dangerous and harassing approaches” toward a fleet of American ships, including the USS Lewis B. Puller, an expeditionary mobile base vessel, and the USS Paul Hamilton, a destroyer. In one case, an Iranian fast boat zipped by within 10 yards of a Coast Guard cutter, the Navy said.

The Revolutionary Guard Corps later acknowledged a tense encounter with the U.S. warships but alleged without evidence that U.S. forces initiated it.

“I have instructed the United States Navy to shoot down and destroy any and all Iranian gunboats if they harass our ships at sea,” Trump said in a tweet.

“Shoot down” implies that the target is airborne. The Iranian vessels in question are surface ships, which the Navy could attack with missiles, torpedoes or other weapons.

In a subsequent tweet, Trump suggested that his administration would adopt a tougher posture toward such incidents than had the Obama administration.

“Sleepy Joe thought this was OK. Not me!” Trump said, referencing former Vice President Joe Biden, now the presumptive Democratic presidential nominee.

The president has a pattern of issuing tough military threats to Iran, often alongside offers to negotiate. He has imposed crippling economic sanctions meant to force Iran to bargain with him over its nuclear program after Trump pulled the United States out of the 2015 pact between world powers and Iran that he deemed too weak.

The White House declined to say Wednesday whether the president has issued a written order or to provide a copy. Pentagon officials did not directly answer when asked by reporters Wednesday whether the military has received an order changing long-standing policies or procedures regarding naval engagements.

“So, the president issued an important warning to the Iranians. What he was emphasizing is all of our ships retain the right of self-defense, and people need to be very careful in their interactions to understand the inherent right of self-defense,” Deputy Defense Secretary David Norquist said when asked about Trump’s tweet.

Gen. John Hyten, vice chairman of the Joint Chiefs of Staff, said that use of deadly force “depends on the situation and what the captain sees” but that he did not want to discuss hypotheticals.

Under current policy, U.S. ships have the right to defend themselves but are expected not to open fire on harassing boats unless required for the safety of U.S. ships and crews.

Trump said Iranian ships should be destroyed for engaging in harassment, which is short of the determination of lethal intent or hostile intent that guide U.S. military responses now.

“You can’t let a boat, a fast boat, get into a position where they can threaten your ship. And I think that every captain at sea understands what that is right now, and we have very specific guidance on how we can use lethal force,” Hyten said during a Pentagon news briefing.

“I go back to – what the president says sends a great message to Iran. That’s perfect. We know how to translate that into our rules of engagement. We don’t talk about rules of engagement in public, but they’re based on the inherent right of self to defense, they’re based on hostile intent and hostile act. That’s all we need in order to take the right action,” Hyten said.

Secretary of State Mike Pompeo also was asked what the tweet meant.

“What he said this morning, and what I know he’s told all of us in leadership inside the government, is ‘take whatever action is necessary to make sure that you can defend and keep our people safe,’ ” Pompeo said.

A senior Navy spokesman, Rear Adm. Charles Brown, said Wednesday that the Navy will continue to follow international laws of armed conflict. Those include guidelines that dictate that a ship’s crew should not exceed the amount of force necessary to repel an attack.

A spokesman for the Iranian armed forces said Trump’s threat is misplaced.

“Today, instead of bullying others, the Americans should put all their efforts toward saving those members of their forces who are infected with coronavirus,” Abolfazl Shekarchi said, according to the Iranian Students’ News Agency.

The Guard also announced Wednesday that it has put a military satellite into orbit for the first time, a move that further exacerbated tensions with the United States.

Pompeo told reporters Wednesday that the launch puts the lie to Iran’s assertion that its space program is for commercial purposes.

“I think every nation has an obligation to go to the United Nations and evaluate whether this missile launch was consistent with that Security Council resolution,” he said, referring to the resolution that endorsed the 2015 nuclear agreement and called on Iran not to test missiles capable of carrying a nuclear warhead.

“I don’t think it remotely is,” he added. “They’ve now had a military organization that the United States has designated as terrorist attempt to launch a satellite.”

The Trump administration has had military clashes with Iran and affiliated groups over the past year. Iran downed a U.S. drone and launched ballistic missiles at facilities housing U.S. personnel in Iraq. The United States, meanwhile, conducted a drone strike in Baghdad in January that killed Maj. Gen. Qasem Soleimani, a top Iranian commander, and has attacked Iranian-linked militias in Iraq.

Encounters at sea have been common at different points in recent years, with naval officials sometimes classifying Iranian approaches as “unsafe and unprofessional.”

The Navy has long adhered to a policy of defending ships with an escalating series of warnings that include attempts to communicate by radio, horn and warning shots.

The Navy said the Iranian ships ignored U.S. communications for an hour. After finally responding to a bridge-to-bridge radio query, the Iranian ships turned away, the Navy said.

In a training scenario observed by The Washington Post in January, a crew aboard the USS Tempest, a 170-patrol craft, practiced how to respond to a harassing speedboat nearby in the Persian Gulf. Even a couple of weeks after Iran launched ballistic missiles at bases in Iraq in retaliation for the U.S. killing of Soleimani, the Navy stressed to sailors the need to follow protocols and thoughtfully escalate the use of force only as needed.

In the wake of provocative actions at sea attributed to Iran, the United States and several partner nations last year established a new mission headquartered in Bahrain, Operation Sentinel, designed to prevent the seizure of commercial ships or attacks on them. Smaller ships like the Tempest are expected to carry out “sentry” duties and respond to problems that arise, and larger vessels such as destroyers use their sensors to watch out for attacks in a “sentinel” role.

Richard Goldberg, who until recently directed efforts to counter Iran at the National Security Council, said the April 15 encounter in the Persian Gulf probably led to “high-level discussions on existing rules of engagement and whether additional steps were needed to strengthen U.S. deterrence” and protect U.S. forces.

“While Iran’s leaders are growing more desperate for sanctions relief and may believe that harassing the U.S. Navy will prompt political pressure on the president to provide sanctions relief, they also know they cannot win a direct military confrontation with the United States,” said Goldberg, who is a senior adviser at the Foundation for Defense of Democracies.

Trump critics said he is risking war, and perhaps doing so to distract from domestic problems, including the administration’s handling of the coronavirus pandemic.

“There is no worse time for a disastrous war than in the middle of a pandemic, with U.S. service members already at heightened risk of contracting covid-19,” said Jamal Abdi, president of the National Iranian American Council.

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The Washington Post’s Missy Ryan contributed to this report.