The U.S. military in Iraq says six American troops have been killed in roadside bombings southwest of Baghdad.

The military says four of the soldiers were killed Sunday when a bomb exploded near a unit responding to a roadside attack that had killed two U.S. soldiers late Saturday.

A separate statement says a U.S. Marine died Sunday in a non-combat-related incident in al-Anbar province.

Meanwhile, Republican U.S. senator and presidential candidate John McCain has made an unannounced visit to Iraq. He said the new U.S. strategy of sending more forces to Iraq is working.

McCain visited Baghdad Sunday with a congressional delegation, including fellow Republican Senator Lindsey Graham. Graham said the U.S. is doing what it should have done from the start by having enough forces on the ground in Iraq.

Graham added that Iraqi officials believe the new strategy is working.

U.S. military officials say one half of the 30,000 additional U.S. troops being sent to Iraq are already in the country, with the rest expected by June.

Britain’s Ministry of Defense said a British soldier died Sunday after being attacked in Basra, in southern Iraq.

Some information for this report was provided by AFP, AP and Reuters.