BAGHDAD – U.S. Ambassador Ryan Crocker on Thursday accused Iran of “pushing very hard” to derail a security agreement that would authorize American troops to remain in Iraq past Dec. 31.

Crocker also speculated that Iran might be tightening its ties to Shiite Muslim militias in Iraq and co-opting them from anti-U.S. cleric Muqtada al-Sadr, who for the past year has ordered his followers to largely refrain from violence. He said Iran has a history of using members of opposition groups in other countries to its advantage.

“I think what we may be seeing is a situation in which these groups or their successors are far more tightly linked to Tehran and perhaps less linked to Sadr,” Crocker said in an interview.

That could mean a resurgence of militia activity if fighters decide the time is right. And coming at a time of enhanced Iraqi government sovereignty here, and with provincial elections planned by Jan. 31 and national elections next year, there is plenty at stake, particularly in the oil-rich south where Shiite parties with strong Iranian links will vie for power.

A U.N. mandate authorizing the U.S. troop presence in Iraq expires at the end of this year. Crocker said he was convinced that Iraq’s government and people would not put up with Iranian meddling following the bloodshed of March and April, when hundreds died in clashes between Iraqi and U.S. forces and Shiite militias.