Baghdad, Iraq ? In the early morning darkness of Monday, Shiite militiamen fired machine guns and rocket-propelled grenades at oncoming U.S. and British troops, who responded by overwhelming the attackers with gunfire and airstrikes that left at least 20 people dead, the U.S. military said.

The firefight in Maysan province came as thousands of U.S. and Iraqi troops pushed into insurgent strongholds around Baghdad as part of an intensified effort to disrupt militants before they can launch attacks in the capital. The operations are bolstered by the full forces of five new U.S. combat brigades that are now on the ground. A priority of the effort is in the rural territory south of Baghdad, near where two of the five brigades are based.

The U.S. military reported Monday that fighter jets on Saturday dropped four “precision-guided bombs” in the Arab Jubour area south of the capital while 1,200 soldiers maneuvered to prevent insurgents from entering Baghdad.

The fighting in Maysan, which abuts the Iranian border in southern Iraq, broke out about 2:30 a.m. in two predominantly Shiite cities – Amarah and Majar al-Kabir – when U.S.-led troops raided areas looking for people believed to be militiamen involved in funneling armor-piercing explosives and fighters from Iran into Iraq. After the clashes started, attack aircraft strafed the areas, wounding at least six people in addition to the dead, the U.S. military said.

No Iraqi soldiers participated in the attack, even though Iraqi officials took responsibility for security in Maysan in April. Both U.S. and British military spokesmen said that the Iraqi government approved the mission, but a Maysan province security official, Latif al-Tamimi, said “occupation forces” conducted the raids without the knowledge of the provincial council. He called for an investigation into the attack because of what he said were civilian casualties.