President Barack Obama's pick to become the top United States military officer has warned Tehran not to underestimate American resolve in responding to Iranian-sponsored attacks on U.S. forces in Iraq.

General Martin Dempsey said Tuesday that Iran's increased arming of Shi'ite militiamen in southern Iraq who are targeting U.S. troops may be designed to trigger a “Beirut-like moment” of mass casualties.

He was alluding to the 1983 suicide bombing of a Marine barracks in Lebanon that killed 241 American servicemen and drove the U.S. out of the country. Dempsey said Iran wants to send a similar message that it has expelled the U.S. from Iraq.

In testimony before a U.S. Senate committee, he said Tehran's alleged pursuit of atomic weapons and its surrogate activities in southern Iraq increase the potential “that Iran will make a serious miscalculation of U.S. resolve.”

Dempsey's remarks underscore growing concern following the killing of 14 U.S. troops in Iraq in June, the highest monthly toll in three years.

The Obama administration's nominee to be the next chairman of the Joint Chiefs of Staff told Congress Iran would be mistaken to believe Washington would “simply allow that to occur without taking serious consideration or reacting to it.”

Dempsey, who has commanded troops in Iraq, said U.S. military leaders there and his Iraqi contacts have shared information with him about Iran's alleged plans. He did not specify which Iraqis made the allegations.

The U.S. is scheduled to withdraw virtually all of its 46,000 remaining forces from Iraq by the end of the year under the terms of a bilateral security pact.

Dempsey said he would favor extending the U.S. troop presence, if Iraq asks.