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.
McCain, a Republican presidential hopeful who was a prisoner of war in Vietnam, acknowledged a difficult task lies ahead in Iraq, but criticized the media 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-Qaida in the western Anbar province.
"These and other indicators are reason for cautious, very cautious optimism about the effects of the new strategy," said McCain, who was leading a Republican congressional delegation to Iraq that included Sen. Lindsey Graham.
McCain, R-Ariz., was combative during the news conference, refusing to respond to a question about whether the U.S. had plans to attack Iran. He also replied testily to a question about remarks he had made in the United States last week that it was safe to walk some Baghdad streets.
"Things are better and there are encouraging signs. I've been here ... many times over the years. Never have I been able to drive from the airport, never have I been able go out into the city as I was today," he said.
"I'm not saying â€˜mission accomplished,' â€˜last throes,' 'dead-enders' or any of that," he said. "I believe that the signs are encouraging, but please don't interpret one comment of mine in any way to indicate that this isn't a long, difficult struggle."
Members of the delegation spoke at a Green Zone news conference after they rode from Baghdad's airport in armored vehicles under heavy guard to visit the city's largest market, which was been hit by bombings including a February attack that killed 137 people. They said the trips were proof that security was improving in the capital. Prominent visitors normally make the trip from the airport to the city center by helicopter.