Washington: At a time when President Barack Obama is under political pressure from congressional Republicans over negotiations to rein in Tehran’s nuclear ambitions, a startling paradox has emerged: Obama is becoming increasingly dependent on Iranian fighters as he tries to contain the Daesh militant group in Iraq and Syria without committing US ground troops.

In the four days since Iranian troops joined 30,000 Iraqi forces to try to wrest Saddam Hussain’s hometown of Tikrit back from Daesh control, US 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 US war planners have been closely monitoring Iran’s parallel war against Daesh through a range of channels, including conversations on radio frequencies that each side knows the other is monitoring. And the two militaries frequently seek to avoid conflict in their activities by using Iraqi command centres as an intermediary.

As a result, many national security experts say, Iran’s involvement is helping the Iraqis hold the line against Daesh advances until US 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 R. Nasr, a former special adviser to Obama who is now dean of the School of Advanced International Studies at Johns Hopkins University. “You can’t have your cake and eat it too — the US strategy in Iraq has been successful so far largely because of Iran.”

It was Iran that organised Iraq’s Shiite militias last August to break a week-long Daesh siege of Amerli, a cluster of farming villages whose Shiite residents faced possible slaughter. US bombs provided support from warplanes.

Administration officials were careful to note at the time that the United States was working in Amerli with its allies — namely Iraqi army units and Kurdish security forces. A senior administration official said that “any coordinating with the Shiite militias was not done by us; it would have been done by the ISF,” a reference to the Iraqi security forces.

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

And last summer, when Daesh militants first captured Mosul and got within striking distance of the Kurdish capital, Arbil, the head of Iran’s Quds Force, Major General Qasim Sulaimani, flew to Arbil with two planes full of military supplies, US and regional diplomats said. The Iranian move helped to bolster Kurdish defences around Arbil, the officials said.

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

Websites supporting the militias circulated photographs of Sulaimani on Wednesday drinking tea on what was said to be the front line, dressed in black and holding his glass in one hand and a floral patterned saucer in the other.

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

“There’s just no way that the US military can actively support an offensive led by Sulaimani,” said Christopher Harmer, a former aviator in the US Navy in the 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 US strategy in Iraq can benefit from Iran’s effort to take back Tikrit from Daesh, even if it is not involved directly. Appearing before the Senate Armed Services Committee on Tuesday, General Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, said 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.”

Big worry

But that is a big worry. In the past — notably just after the withdrawal of US 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 Daesh militants.

This week, Republican lawmakers warned that Iran’s influence in Iraq would increase with the Tikrit offensive.

“We share the president’s goal to degrade and defeat Daesh,” Senator John McCain, Republican, Arizona, and Senator Lindsey Graham, Republican, South Carolina, said in a statement on Tuesday. “But success in this mission will not be achieved by capitulating to Iran’s ambitions for regional hegemony.”

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

“By this stage, everybody who observed what happened in Iraq with [Daesh] should know that the main influencer in Iraq is Iran,” he said in a telephone interview on Wednesday. “That’s an unpopular perception in the United States, after spending so much money and lives lost in the conflict, but it’s reality.”

Shroder 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 Daesh is Iran.

Rafid Jaboori, the spokesman for Al Abadi, the Iraqi prime minister, said in an interview on Wednesday that Iraq had urged the United States and Iran not to play out their bilateral conflict in Iraq’s battle against Daesh.

“So far in general there was no clash within the two,” Jaboori said.

He drew a comparison to the Second World War.

“Countries with different ideologies, different priorities, different systems of government, cooperated to defeat the Nazis,” he said. “It’s foreseeable that we see countries which might not get along very well in terms of their bilateral relations working to help Iraq to defeat this threat.”