ISLAMABAD - U.S. senators on Friday defended American drone aircraft strikes in ally Pakistan, an issue likely to become more volatile if Washington intensifies the attacks to hunt down enemies after the bombing of CIA agents in Afghanistan.

Pakistan officially objects to the attacks on suspected al Qaeda and Taliban militants along its border with Afghanistan, saying they violate its sovereignty.

And Islamabad has pushed Washington to provide it with the drones to allow it to carry out its own attacks on Taliban insurgents, a move that could ease widespread anti-American sentiment in Pakistan.

We don't agree on every issue. We believe that, as I have stated and as our government has stated, that it is one of many tools that we must use to try to defeat a very determined and terrible enemy, said U.S. Senator John McCain.

The United States has stepped up its attacks with pilotless drone aircraft attacks in Pakistan since a double agent blew himself up at a U.S. base in Afghanistan on December 30, killing seven CIA agents.

A drone strike on Friday evening, the sixth in the lawless North Waziristan region on the Afghan border since December 30, killed two militants, Pakistani security officials said.

The attack on the CIA was a huge intelligence failure and will pile pressure on the United States to kill high-profile militants based along the Afghan-Pakistan frontier.

The United States sees the drones as a highly effective weapon in a global hub for militants. The strikes have killed some prominent al Qaeda militants.

Many al Qaeda and Taliban members fled to northwestern Pakistan's ungoverned ethnic Pashtun belt after U.S.-led forces ousted Afghanistan's Taliban government in 2001. From there they have orchestrated insurgencies in Afghanistan and Pakistan. Pakistan has not objected to drone strikes that have killed militants fighting the Pakistani state, such as Pakistan Taliban chief Baitullah Mehsud.

But Pakistan does oppose strikes on strategic regional assets such as the Afghan Haqqani militant group, which had ties with Pakistan's ISI spy agency and would give it leverage in Afghanistan if the country is gripped by chaos again.

STRATEGIC COMPLEXITIES

At the same time the al Qaeda-linked Haqqani group is high on the U.S. hit list, and speculation is growing it may have been linked to the bomb attack on the CIA, illustrating the complexities and sensitivities in U.S.-Pakistani ties.

The drone issue was raised when a delegation of U.S. senators led by McCain met President Asif Ali Zardari on Thursday. They have also met the prime minister, as well as army chief general Ashfaq Kayani. He is the pivotal figure because the military makes security decisions and effectively sets foreign policy.

Drone attacks are a politically charged issue between the United States and Pakistan, which Washington sees as the front-line state in its war against the Taliban in Afghanistan.

Pakistan fears the strikes could undermine efforts to deal with militancy because the civilian casualties inflame public anger and bolster support for the fighters.

Asked if he had the same concerns, McCain told reporters:

There are elements operating in Pakistan that if allowed to do so would go to Afghanistan and kill Americans and destroy that government and re-establish Afghanistan as a base for attacks on the United States and our allies. That's what I understand.

The United States carried out 51 drone air strikes in Pakistan last year, killing about 460 people, including many foreign militants, according to a tally of reports from Pakistani officials and residents.

McCain suggested no other options were under consideration in the event that the drone strikes failed to deliver. Asked if he would support U.S. ground operations on Pakistani soil, McCain said he had never been briefed on that.

I think it would have to be done in coordination and in agreement with the Pakistani government and military, he said.

Pakistan's reluctance to go after the Haqqani network, whose leader worked with the CIA in the 1980s against Soviet occupation troops in Afghanistan, has strained ties with Washington.

The U.S. embassy has accused Pakistan of taking provocative action and making false allegations against U.S. personnel. U.S. officials say Pakistan is also stalling their visa applications.

We would like to see it resolved, said McCain.

Al Qaeda's Afghan wing claimed responsibility for the suicide bombing, the second-most deadly attack in CIA history, saying it was revenge for the deaths of militant leaders, including Mehsud, who was killed in a drone attack. His death had not eased a raging Taliban insurgency gripping Pakistan.

(Additional reporting by Faisal Aziz in Karachi; Editing by Robert Birsel)

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