Four American troops were killed by bombs and mortars in three separate incidents in Iraq, the U.S. military confirmed on Friday. A roadside bomb killed one soldier and injured another when it went off by a convoy near Baquba early on Friday, and a second soldier was killed trying to defuse a bomb outside the town, Captain Jefferson Wolfe said. Earlier, Iraqi resistance fighters shelled a base in Baqouba, some 50 kilometers northeast of Baghdad, killing two U.S. troops and injuring four, the US military said Friday. The fighters fired mortars that hit the base around 6:15 p.m. on Thursday, Maj. Josslyn Aberle of the 4th Infantry Division said. Two troops were evacuated for medical care but died of their injuries, she said, according to The AP. The four others did not have life-threatening injuries. Meanwhile, Japan dispatched its first military unit for a humanitarian mission to Iraq on Friday, media reports said. The advance air force contingent of more than 40 personnel was planned to split up and take commercial flights to Kuwait from Tokyo international airport. Kyodo News agency and NHK television said the first flight carrying some of the personnel left Friday morning. At least one other flight was expected to leave later in the day. **PHOTO CAPTION*** U.S. soldiers of 1st Battalion, 22nd Regiment, (1-22) of the 4th Infantry Division, play cards onboard their Humvee in Tikrit, Iraq, Friday, Dec. 26, 2003. (AP Photo/Gregorio Borgia)