McCain met with Deputy Prime Minister Barham Saleh and planned to meet with Gen. David Petraeus, the top U.S. commander in Iraq, according to the U.S. Embassy. Further details of McCain's visit, which had been anticipated, were not being released for security reasons, the embassy said.
Before leaving the United States, McCain, one of the foremost proponents of the March 2003 U.S.-led invasion, said the trip to the Middle East and Europe was for fact-finding purposes, not a campaign photo opportunity.
But he expressed public worries that militants in Iraq might try to influence the November general election.
"Yes, I worry about it," he said, responding to a question during a campaign appearance in Pennsylvania. "And I know they pay attention, because of the intercepts we have of their communications."
McCain, the senior Republican on the Senate Armed Services Committee, was accompanied by Sens. Joe Lieberman, an independent from Connecticut, and Lindsey Graham, R-S.C., two top supporters of his presidential ambitions.
The weeklong trip will take McCain to Israel, Britain and France, and include his first meeting with British Prime Minister Gordon Brown. He also is expected to meet with French President Nicolas Sarkozy, Israeli Prime Minister Ehud Olmert and other Israeli officials.
His focus in Iraq was thought to be the drop in sectarian violence and U.S. and civilian casualties since last summer. Exactly what was discussed, however, remained unclear since numerous telephone calls to aides traveling with McCain went unanswered.
Elsewhere, Kurds in northern Iraq commemorated the anniversary of the chemical weapons attack in Halabja, near the Iranian border, with solemn observances. The streets were empty and heavily patrolled by Iraqi security forces.
Saddam Hussein ordered the 1988 attack as part of a scorched-earth campaign to crush a Kurdish rebellion in the north, which was seen as aiding Iran near the end of its war with Iraq. Saddam was executed for other crimes against humanity before he could face trial for the attacks.
McCain's trip to Iraq is his eighth. Last November, he met with Prime Minister Nouri al-Maliki during the U.S. Thanksgiving holiday.
On a visit last April, the Arizona senator criticized news reports he said focused unfairly on violence, and said Americans were not getting a "full picture" of progress in the security crackdown in the capital.
McCain was combative toward reporters' questions in the heavily guarded Green Zone, and responded testily to a question about his comment that it was safe to walk some Baghdad streets. He later acknowledged traveling with armed U.S. military escorts.
Violence has dropped throughout the capital since, with an influx of some 30,000 additional U.S. soldiers sent to Iraq last year. The U.S. military has said attacks have fallen by about 60 percent since last February.
Still, violence continues in some parts of the country, according to reports from police officials who spoke on condition of anonymity because they weren't authorized to speak to the media.
On Sunday, a parked car bomb exploded in western Baghdad's Mansour neighborhood, killing one person and wounding two others. Two civilians and nine others were wounded in Mosul when a suicide bomber detonated his explosive vest, police said. A roadside bomb killed another person in the northwestern city.
Just outside Baqouba, the capital of restive Diyala province, three people were killed in clashes between police and a faction of radical Shiite cleric Muqtada al-Sadr's Mahdi Army, police said. In the city itself, gunmen killed a city hall employee, police said.
Police also found the bullet-riddled bodies of at least 16 people in Baghdad, Muqdadiyah, Mosul and the southern cities of Basra and Kut, where Shiite militia violence has been on the rise.
In Washington, two of McCain's colleagues who support Democrats for president, said senators - including candidate McCain - have the right to visit various parts of the world.
But, said Sen. Dianne Feinstein, D-Calif: "I think it would have probably have been better if he took members who were not so closely identified with his campaign. But this is indicated to be a congressional visit.
"Obviously the world's going to watch it, and we'll know whether it's exploited for other reasons. I don't believe it will be, but we'll see," Feinstein, who supports New York Sen. Hillary Rodham Clinton for president, said on CNN's "Late Edition." She appeared with Sen. Patrick Leahy, D-Vt., who supports Illinois Sen. Barack Obama.