The clashes Thursday indicate U.S. forces were drawn more deeply into a broad offensive Iraqi Prime Minister Nouri al-Maliki undertook in the southern city of Basra earlier against rouge militias, The Washington Post reported Friday.

The Mehdi Army of cleric Moqtada Sadr, a Shiite rival of Maliki, seems to have absorbed the brunt of the attacks in Basra, and fighting has spread to other southern cities and parts of Baghdad, The Post said.

Maliki implemented the offensive without consulting his U.S. allies, White House officials said. With little U.S. presence in the south and British forces in Basra confined to an air base outside the city, one administration official told the Post, "(We) can't quite decipher" what's happening.

Three rival Shiite groups have been trying to position themselves in Basra to dominate recently approved provincial elections.

In Baghdad, Post reporters said they saw four U.S. Stryker armored vehicles in Sadr City and heard the noise of American weapons and Mehdi Army's AK-47s and rocket-propelled grenades.