ISLAMABAD, (Reuters) – U.S. senators defended on Friday American drone aircraft strikes in 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 drone attacks, saying they violate its sovereignty. And it has pushed Washington to provide it with the drones to allow it to carry out its own attacks.

“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.

There have been a high number of pilotless drone aircraft attacks in Pakistan since a double agent blew himself up at a U.S. base in Afghanistan on Dec. 30, killing seven CIA agents. That blast, which marked a huge CIA intelligence failure, will pile pressure on the U.S. military to kill high-profile militants based along the Afghan-Pakistan frontier.

Drones are seen by the United States as one of the most effective weapons to achieve that goal in a global hub for militants, including top al Qaeda and Taliban figures.

The strikes have killed some prominent al Qaeda militants.

Pakistan has not objected to ones that have killed militants fighting the Pakistani state, such as Pakistan Taliban chief Baitullah Mehsud.

It does oppose strikes on what it sees as strategic regional assets such as the Afghan Haqqani militant group. Pakistan believes the group can give it leverage in Afghanistan if the country is gripped by chaos again.

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 subject of drones was raised when a delegation of U.S. senators led by McCain met Pakistani President Asif Ali Zardari on Thursday.

The delegation also met Prime Minister Yusuf Raza Gilani, as well as army chief general Ashfaq Kayani. He is the most important official because the military makes security decisions and effectively sets foreign policy.

Drone attacks are a politically charged issue between the United States and ally 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 worried civilian casualties would give the United States’ enemies a propaganda boost and that the drone strikes raise the issue of sovereignty, McCain told reporters: “I understand very well that 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.”

McCain suggested there were no other options under consideration if the drone strikes failed to deliver enough results.

Asked if he would support U.S. ground operations on Pakistani soil, McCain said said he had never been briefed that that was under consideration. “Very frankly, 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. “It’s a point of friction. We would like to see it resolved. We would like to see the visas granted that are necessary for our embassy to do our job,” said McCain.

Al Qaeda’s Afghan wing has 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 has not eased Pakistan Taliban bombings which have killed hundreds in retaliation for a security offensive.

At least six people were killed in Karachi on Friday when explosives being stored in an suspected militant hideout apparently went off accidently, police said.