Islamabad, Pakistan - Two leading U.S. senators attempted Friday to depict U.S.-Pakistani relations as a crucial, permanent friendship, but their brief visit to the Pakistani capital highlighted tensions between the anti-terrorist allies, especially a sharp disagreement over strikes by unmanned aircraft against suspected Taliban and al-Qaida targets.

"Friends don't always agree on every issue," Sen. John McCain, R-Ariz., said at a news conference in Islamabad, adding that the United States will "try to find common ground" with Pakistani leaders on the drone issue but that "we have to do everything we feel is necessary to protect Americans from the attacks of terrorists who may be based here."

On Thursday, Pakistani President Asif Ali Zardari asked McCain and Sen. Joe Lieberman, D-Conn., who traveled here with two other senators, to seek a halt to the drone attacks. He said they are undermining domestic support for the war against Islamist militants.

Washington has stepped up its use of the controversial strikes near the Afghan border since the suicide bombing at a U.S. base in eastern Afghanistan on Dec. 30 that killed seven CIA officers and contractors.

In the latest raid Friday, suspected U.S. missiles killed four people and injured three in the North Waziristan tribal area, the sixth attack in the region in a little more than a week, the Associated Press reported. Two Pakistani intelligence officials said a pair of missiles struck a house and a vehicle in a village near the town of Miran Shah. They did not identify the victims.

Lieberman stressed the importance of bilateral cooperation against Islamist extremism and tried to reassure Pakistanis that the United States will not abandon them as it did after Soviet troops withdrew from Afghanistan in 1989.