BAGHDAD, (AP) – Explosions rocked Iraq’s capital on Monday as Vice President Dick Cheney and Republican presidential hopeful Sen. John McCain visited ahead of the fifth anniversary of the U.S.-led invasion.

Helicopter gunships circled over central Baghdad and the heavily fortified Green Zone, but no details were immediately available on the cause of the explosions.

McCain, the Republican party’s likely presidential nominee who has linked his political future to U.S. military success in Iraq, met Monday with Prime Minister Nouri al-Maliki shortly before the Iraqi leader began talks with Cheney.

The Arizona senator stressed the importance of a U.S. commitment to Iraq and warned a U.S.-Iraqi military operation to clear al-Qaeda from its last urban stronghold of Mosul will be “very difficult and very important.”

McCain, who arrived in Iraq on Sunday, told reporters that he also discussed with the Shiite leader the need for progress on political reforms, including laws on holding provincial elections and the equitable distribution of Iraq’s oil riches.

McCain said he and Iraqi government officials had reviewed the security situation in Baghdad, although more details were not available.

Cheney landed at Baghdad International Airport, then flew by helicopter for talks with U.S. and Iraqi officials. It is Cheney’s third vice presidential trip to Iraq where 160,000 American troops are deployed and the U.S. death toll is nearing 4,000.

The U.S. Embassy in Baghdad said it could not confirm reports of a rocket attack on the Green Zone after Cheney’s arrival. “I’m not aware of any,” embassy spokeswoman Mirembe Nantongo said.

Violence has dropped throughout the capital with an influx of some 30,000 additional U.S. soldiers as well as a Sunni revolt against al-Qaeda and a cease-fire by radical Shiite cleric Muqtada al-Sadr’s Mahdi Army militia. The U.S. military has said attacks have fallen by about 60 percent since last February.

McCain met with Deputy Prime Minister Barham Saleh on Sunday and planned to meet with Gen. David Petraeus, the top U.S. commander in Iraq, the U.S. Embassy said. Further details of the visit were not released for security reasons, the embassy said.

Before leaving the United States, McCain, who was making his eighth trip to Iraq, said the tour to the Middle East and Europe was for fact-finding purposes, not a campaign photo opportunity.

McCain, the senior Republican on the Senate Armed Services Committee, was accompanied by Sens. Joe Lieberman, an independent, and Republican Lindsey Graham, two top supporters of his presidential ambitions. The weeklong trip will take McCain to Israel, Britain and France.

In other violence Monday, police said they found the bodies of three members of a U.S.-allied group fighting al-Qaeda in Udaim, 70 miles north of Baghdad. Members of the mostly Sunni groups have been increasingly targeted by suspected al-Qaeda members seeking to derail the recent security gains.