BAGHDAD » A roadside bomb killed two U.S. soldiers Thursday outside the main American military base in Baghdad in what U.S. officials said appeared to be another attack by Shiite militias hoping to drive U.S. troops out of Iraq.

The attack follows the deadliest month for U.S. troops in Iraq in two years. Fifteen U.S. soldiers died in June, nearly all in attacks by Shiite militias.

The 46,000 U.S. troops currently stationed in Iraq are to leave by year’s end under a 2008 withdrawal agreement. However, the White House is offering to keep up to 10,000 American soldiers in the country beyond that deadline, if asked by Iraq, to help stabilize the country’s still-shaky security and keep Iran from becoming too cozy with Baghdad officials.

Thursday’s bomb, which was detonated near a checkpoint outside Victory Base Camp, was a powerful armor-piercing explosive known as an EFP, according to two military officials with knowledge of the attack.

EFPs typically are designed and outfitted in Iran, and are a trademark of the Shiite militia headed by anti-U.S. cleric Muqtada al-Sadr.

The U.S. officials spoke on condition of anonymity because the details have not been officially released.

A spokesman for the U.S. military in Iraq did not have an immediate comment about the attack.

Victory Base Camp serves as the headquarters of the U.S. military in Iraq, and is home to thousands of American soldiers and contractors.

The slain soldiers were not identified pending notification of their relatives, and no other details were immediately available.

While the White House is considering keeping some U.S. soldiers in Iraq beyond this year, some U.S. officials oppose an extension. They fear the recent uptick in American deaths will continue if soldiers remain in Iraq — a risk they say is not worth taking.

The most recent attack brings the number of U.S. troops who have died in Iraq to 4,471 since the 2003 invasion, according to an count.

Iraq and Iran have the only two Shiite-led governments in the region. U.S.-allied Arab states, particularly Saudi Arabia and its Gulf neighbors, are concerned about what they perceive to be Iran’s expanding influence ahead of a planned departure of U.S. forces from Iraq. The Sunni-led monarchies of the Gulf have also accused Tehran of encouraging Shiite-led uprisings in Bahrain and other Gulf states.