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WASHINGTON — The Obama administration said Thursday the Syrian government has likely used chemical weapons on a small scale against its own people, but it stopped short of threatening military action against President Bashar Assad.

In a letter to key lawmakers, the administration said U.S. intelligence agencies “assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin.”

The finding was based in part on analysis of “physiological samples,” apparently referring to hair and blood from victims.

Despite the caveats, the disclosure puts President Obama under new pressure to respond because it is the first time the United States has joined other countries in suggesting the Assad government is likely to have deployed chemical weapons during the two-year Syrian civil war.

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A senior administration official acknowledged that any use of chemical weapons in Syria would cross the “red line” declared by Obama many times in recent months in warnings to Assad. The official, who spoke on condition of anonymity, said the administration was waiting for a “definitive judgment.”

Instead of outlining specific action, the administration reaffirmed its support for a comprehensive U.N. investigation inside Syria to gather evidence. Assad has refused to admit the U.N. team amid a dispute over the scope of the investigation.

The U.S. disclosure brought a swift response from Congress.

Sen. John McCain, R-Ariz., said it was “pretty obvious that a red line has been crossed.” House Speaker John Boehner, R-Ohio, accused the administration of outsourcing national security to the United Nations.

Senior Democrats also voiced concerns. Senate Foreign Relations Committee Chairman Robert Menendez, D-N.J., said the Syrian government had “crossed a red line by using chemical weapons, which forces us to consider all options as to how we act to influence the balance of the conflict.”

Obama confronts no easy choices and all are fraught with risk, experts said.

“This is a case where there is nothing but bad options,” said Anthony Cordesman, an expert with the Center for Strategic and International Studies.

One overriding consideration: Anything the United States does to weaken the Assad government could aid Islamist groups that have emerged as the most effective fighting forces of the divided and poorly organized Syrian opposition.

Those groups include the Nusra Front, a branch of al-Qaida in Iraq, that has been joined by foreign jihadists and seeks to impose Taliban-style Islamic rule.

Moreover, Americans are weary of foreign entanglements after the more than eight-year U.S. occupation of Iraq and the 12-year war in Afghanistan. Even among the most ardent proponents of greater U.S. help for the Syrian opposition, there is no support for sending in U.S. troops.

U.S. commanders have laid out a range of possible options for military involvement in Syria, but they have made it clear any action would likely be either with NATO backing or with a coalition of nations similar to the NATO-led overthrow of Libyan dictator Col. Moammar Gadhafi in 2011.

Military options could include establishing a no-fly zone or a secured area within Syria, launching airstrikes by drones and fighter jets, or sending in tens of thousands of ground forces to secure the government’s chemical-weapons caches.

Setting up a no-fly zone over Syria would present a greater challenge than it did in Libya because Syria has a more sophisticated and robust air-defense system. Crippling it would require jamming the radars and taking out the missile sites, or possibly even using some type of cyberattack to interfere with the system.

Some senators have pressed for the U.S. to set up a narrow, so-called safe zone in Syria along its border with Turkey where citizens could go and be safe. To do so would also require neutralizing Syria’s air defenses.

The U.S. conclusions about chemical weapons in Syria echoed those of Britain, France and Israel, which have suggested in recent days that forces loyal to Assad have probably used sarin. According to senior diplomats and officials briefed on the British and French accounts, the evidence included soil samples and witness interviews that point toward the use of nerve agents in and around the cities of Aleppo, Homs and Damascus.

The U.S. assessment, compiled from many intelligence agencies and finalized in recent days, eased the official skepticism that greeted the Israeli assertions just two days earlier.

However, questions surfaced among experts and within the U.S. government about the strength of the evidence showing chemical weapons have been used.

Secretary of Defense Chuck Hagel said in Abu Dhabi that the conclusion had been made in the previous 24 hours.

“We cannot confirm the origin of these weapons, but we do believe that any use of chemical weapons in Syria would very likely have originated with the Assad regime,” he said. “As I’ve said, this is serious business. We need all the facts.”

Experts say the reports should be met with some skepticism because of the small amount of sarin that was found, the lack of widespread deaths and injuries, and inconclusive U.S. intelligence assessments.

A person familiar with the issue, who asked not to be further identified, said that only a minuscule trace of a “byproduct” — a toxic residue left behind after use of a nerve agent, and which he did not identify — had been found in a soil sample.

“They found trace amounts of a byproduct in soil, but there are also fertilizers that give out the same byproduct,” the person said. “It’s far from conclusive.”

So far, the United States has provided hundreds of millions of dollars in aid to Syrians.

Syria possesses one of the world’s largest inventories of chemical weapons, including sarin and other nerve agents banned by an international treaty that Assad’s government has refused to sign.

The administration said Assad remains in control of the weapons, but U.S. officials have expressed concerns the material could fall into the hands of extremists within the Syrian opposition or Hezbollah, the Lebanese Shiite militant group fighting alongside Syrian troops.

Pentagon officials have said it could take tens of thousands of U.S. troops to secure Syria’s chemical weapons as long as the civil war is raging.

If the U.S. military intervened in Syria, it would almost certainly face attacks from Assad’s forces. Rebel fighters allied with al-Qaida also would pose a threat.

Bombing Assad’s chemical-weapons stockpile could be even riskier, military analysts said. Airstrikes could easily backfire by dispersing nerve gases and other chemicals over populated areas.

Last week, the Pentagon said additional U.S. troops would go to Jordan to help cope with a flood of refugees crossing the border from Syria, but also to plan for possible responses to any outbreak of chemical warfare. The new troops will bring the U.S. total to about 200.

The civil war in Syria has killed more than 70,000 people and made refugees of more than 1 million Syrians. Despite the humanitarian toll, Obama has been wary of deepening U.S. military involvement. The administration, however, has widened defensive and humanitarian support for the Syrian rebels.

U.S. officials invoked the mistakes of the Iraq war as they urged caution. The George W. Bush administration deposed Saddam Hussein in 2003, based on notoriously erroneous intelligence that he possessed weapons of mass destruction.

“Don’t take from this that this is an automatic trigger,” said a senior defense official, speaking on the condition of anonymity. “We have seen very bad movies before when intelligence is perceived to have driven policy decisions that, in the cold light of day, have been proven wrong.”

Material from McClatchy Newspapers and The Associated Press is included in this report.

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