BAGHDAD — After a heavily guarded trip to a Baghdad market, Senator McCain insisted yesterday that an American-Iraqi security crackdown in the capital was working and said Americans lacked a “full picture” of the progress. The U.S. military later reported six soldiers were killed in roadside bombings southwest of Baghdad.

Four soldiers were killed responding to the blast that killed the first two, the military said. Britain, meanwhile, announced that one of its soldiers had been shot to death in southern Iraq — its 104th combat casualty since the war started four years ago.

Mr. McCain, a Republican presidential hopeful who was a prisoner of war in Vietnam, acknowledged a difficult task lies ahead in Iraq, but he criticized the press for not giving Americans enough information about the recent drop in execution-style sectarian killings, the establishment of security posts throughout the city, and Sunni tribal efforts against Al Qaeda in the western Anbar province.

“These and other indicators are reason for cautious, very cautious optimism about the effects of the new strategy,” said Mr. McCain, who was leading a Republican congressional delegation to Iraq that included Senator Graham.

The congressmen, who wore body armor during their hour-long shopping excursion, said they were impressed with the resilience and warmth of the Iraqi people, some of whom would not take money for their souvenirs. They were accompanied by the top U.S.commander in Iraq, General David Petraeus.

While the capital has seen a recent dip in violence as extra U.S. and Iraqi troops have flooded the streets, an Iraqi military spokesman said that militants fleeing the crackdown have made areas outside the capital “breeding grounds for violence,” spreading deadly bombings and sectarian attacks to areas once relatively untouched.