US Vice-President Dick Cheney is in the Iraqi capital Baghdad for a previously unannounced visit, ahead of the fifth anniversary of the US-led invasion.
Dick Cheney was briefed on arrival in Iraq by General David Petraeus
Mr Cheney has had a classified briefing with the top US commander in Iraq, Gen David Petraeus, and the US ambassador to the country, Ryan Crocker.
During the visit a series of explosions shook Baghdad, killing at least four.
North of the capital, police said they found the bodies of three members of a US-backed neighbourhood police group.
After a meeting with Iraqi Prime Minister Nouri Maliki, Mr Cheney said it was significant to be in Iraq five years after a campaign "that liberated the people of Iraq from Saddam Hussein's tyranny and launched them on the difficult but historic road to democracy".
The visit came as Mr Cheney began a 10-day tour of the Middle East.
He is due to hold talks with leaders in Oman, Saudi Arabia, Israel, the West Bank and Turkey.
Oman had been announced as the first scheduled stop on Mr Cheney's tour, but on Sunday night he left the vice presidential plane, Air Force Two, in the UK and boarded a C-17 flight to the Iraqi capital.
Mr Cheney landed at Baghdad International Airport before flying by helicopter to the Green Zone.
It is his third vice-presidential trip to Iraq, where 160,000 American troops are deployed and the US death toll is nearing 4,000.
The expected Republican presidential candidate, John McCain, is also in Iraq, on the second day of his visit.
He said it was important that the US maintained its commitment to Iraq.
"We recognise that al-Qaeda is on the run, but they are not defeated," Mr McCain said after a meeting with Mr Maliki.
"Al-Qaeda continues to pose a great threat to the security and very existence of Iraq as a democracy. So we know there's still a lot more of work to be done."
Mr McCain said he had also discussed the need for Iraqi political reform, including measures to allow for provincial elections and for sharing out Iraq's oil wealth.
He is continuing meetings with senior American and Iraqi officials, before heading to other countries in the Middle East and Europe.
Mr Cheney is also expected to discuss Iraqi reforms.
It is unclear whether Mr Cheney and Mr McCain will cross paths in Iraq.