Many national-security experts say Iran’s involvement in Iraq is helping the Iraqis hold the line against Islamic State group advances until U.S. military advisers are finished training Iraq’s underperforming armed forces.

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WASHINGTON —

At a time when President Obama is under pressure from congressional Republicans over negotiations to rein in Iran’s nuclear ambitions, a startling paradox has emerged: Obama is becoming increasingly dependent on Iranian fighters as he tries to contain the Islamic State group in Iraq and Syria without committing U.S. ground troops.

Since Iranian troops joined 30,000 Iraqi forces Monday to try to wrest Saddam Hussein’s hometown of Tikrit back from Islamic State control, U.S. officials have said the United States is not coordinating with Iran, one of its fiercest global foes, in the fight against a common enemy.

That may be technically true. But U.S. war planners have been closely monitoring Iran’s parallel war against the Islamic State group, also known as ISIL and ISIS, through a range of channels, including conversations on radio frequencies that each side knows the other is monitoring. The two militaries frequently seek to avoid conflict by using Iraqi command centers as an intermediary.

As a result, many national-security experts say, Iran’s involvement is helping the Iraqis hold the line against Islamic State group advances until U.S. military advisers are finished training Iraq’s underperforming armed forces.

“The only way in which the Obama administration can credibly stick with its strategy is by implicitly assuming that the Iranians will carry most of the weight and win the battles on the ground,” said Vali Nasr, a former special adviser to Obama who is now dean of the School of Advanced International Studies at Johns Hopkins University. “The U.S. strategy in Iraq has been successful so far largely because of Iran.”

It was Iran that organized Iraq’s Shiite militias in August to break a weeklong Islamic State siege of Amerli, a cluster of farming villages whose Shiite residents faced possible slaughter by the Islamic State militants who espouse an extreme Sunni ideology. U.S. bombs provided support from warplanes.

It was also Iran’s Quds Force that backed Shiite militias and Iraqi security forces in November to liberate the central city of Baiji from the Islamic State group, breaking the siege of a nearby oil refinery. A month later, Islamic State forces took back a part of the city.

Last summer, when Islamic State militants first captured Mosul and got within striking distance of the Kurdish capital, Irbil, the head of Iran’s Quds Force, Maj. Gen. Qassim Suleimani, flew to Irbil with two planes full of military supplies, U.S. and regional diplomats said. The Iranian move helped to bolster Kurdish defenses around Irbil, the officials said.

In Tikrit this week, Iranian-backed Shiite militia leaders said their fighters made up more than two-thirds of the pro-government force of 30,000. They also said that Suleimani, the Iranian spymaster, was helping to lead from near the front line.

Websites supporting the militias circulated photographs of Suleimani on Wednesday drinking tea on what was said to be the front line.

The presence of Suleimani — a reviled figure in U.S. security and military circles because he once directed a deadly campaign against U.S. forces in Iraq — makes it difficult for the United States to conduct airstrikes to assist in the Tikrit operation, as it might like, foreign-policy experts said.

“There’s just no way that the U.S. military can actively support an offensive led by Suleimani,” said Christopher Harmer, a former U.S. Navy aviator in the Persian Gulf who is now an analyst with the Institute for the Study of War. “He’s a more stately version of Osama bin Laden.”

But the U.S. strategy in Iraq can benefit from Iran’s effort to take back Tikrit from the Islamic State, even if it is not involved directly. On Tuesday, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told the Senate Armed Services Committee that the involvement of Iranian-backed Shiites in Tikrit could be “a positive thing,” provided it did not exacerbate sectarian tension.

“This is the most overt conduct of Iranian support, in the form of artillery and other things,” Dempsey said. “Frankly, it will only be a problem if it results in sectarianism.”

But that is a big worry. In the past — notably just after the withdrawal of U.S. troops from Iraq in 2011 — Shiite militias have been accused of atrocities against Sunnis. And in January, Prime Minister Haider al-Abadi ordered an investigation into accusations that Shiite militiamen massacred 70 people in Diyala province after pro-government forces expelled Islamic State militants.

Beyond that, the closer the United States appears to get to Iran, the more fretful Sunni Arab allies become. On Thursday, Secretary of State John Kerry sought to reassure Saudi Arabia and other Persian Gulf states that nuclear negotiations would not lead the Obama administration to let down its guard against Iranian influence in the region. But Prince Saud al-Faisal, the Saudi foreign minister, used the opportunity to complain about the Tikrit operation, saying Iran’s role there was an indication of what he called Iran’s “hegemonic” tendencies.

Landon Shroder, an intelligence analyst for corporations in Iraq who was in Baghdad last summer when Mosul fell, said the worry that Iran will gain influence in Iraq ignores that Iran’s Shiite government is already a key Iraqi ally.

“By this stage, everybody who observed what happened in Iraq with the Islamic State should know that the main influencer in Iraq is Iran,” he said in a telephone interview Wednesday. He said that at the moment, the only force with the ability to bring Kurdish troops, the Iraqi army and the Shiite militias together to fight the Islamic State is Iran.