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

In the four days since Iranian troops joined 30,000 Iraqi forces to try to wrest Saddam Hussein’s hometown of Tikrit back from Islamic State control, American 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 American war planners have been closely monitoring Iran’s parallel war against the Islamic State, also known as ISIS or ISIL, 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 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 advances until American military advisers are finished training Iraq’s underperforming armed forces.