BAGHDAD — A roadside bomb killed two U.S. soldiers today 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 bomb in Baghdad, 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.

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 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.