ISLAMABAD — At least 45 suspected militants were killed by missiles launched by U.S. drone aircraft in Pakistan's northwest in less than 24 hours, local intelligence officials said Tuesday.

Coming a day after Washington announced an $800 million delay in military assistance amid worsening U.S.-Pakistan ties , the attacks could exacerbate tensions between the two uneasy allies in the war against militants.

The attacks started on Monday night, when remotely piloted drones fired nine missiles into a militant compound and at a vehicle in North Waziristan, killing 25 suspected insurgents, local intelligence officials said.

Another strike hours later in South Waziristan killed five suspected militants.

Then on Tuesday morning, a drone fired two missiles at another compound in North Waziristan.

"The missiles were fired as militants sitting in a vehicle were entering into a house used by them as a hideout," an intelligence official told Reuters, adding that 15 militants were killed in the strike. "The house is on fire."

There was no independent confirmation of the death tolls, and militants often dispute official death figures.

It was the second-largest death toll in a day in the unacknowledged U.S. drone campaign against militants in Pakistan's northwest. In June 2009, about 70 suspected militants were killed in a drone attack in South Waziristan.

Militant sanctuaries
Most of the strikes have been concentrated in South and, especially, North Waziristan, mountainous tribal regions on the Afghan border that shelter militant groups friendly with Pakistan but who are attacking U.S. and NATO forces in Afghanistan.

Washington has been pushing Pakistan to mount an offensive against these militant sanctuaries for years, but Pakistan has resisted, saying it must consolidate its gains against Taliban militants elsewhere first. The United States has stepped up drone attacks in response to Pakistan's perceived recalcitrance.

Drone strikes have become one of the most contentious issues in the U.S.-Pakistan relationship. While Pakistan has always publicly opposed the strikes, privately it allowed them and cooperated with the United States determining targets.

But since the May 2 commando raid that killed Osama bin Laden, which Pakistan considers a grievous breach of sovereignty, the powerful head of the army, General Ashfaq Kayani, has called for a halt.

No one from the military or government was available for comment.

Joint intelligence operations between Pakistan and the United States were suspended in late January, after a CIA contractor killed two Pakistanis in Lahore.

It is unknown if operations were ever restarted, and Pakistan's Directorate of Inter-Services Intelligence (ISI), the powerful spy agency, has said the United States has its own targeting information and no longer relies on Pakistani intelligence.

More than 135 militants have been killed since the beginning of June in drone attacks, according to Reuters figures and based on statements from local intelligence officials.

The United States this week said it was holding back $800 million in military aid to Pakistan in a show of displeasure over Pakistan's cutback of U.S. military trainers, limits on visa for U.S. personnel and other bilateral irritants.

The Pakistan military said on Monday it could do without the U.S. assistance by depending on its own resources or turning to "all-weather friend" China.

Former Pakistani officials and analysts warned Monday that the suspension of aid could up hurting Washington more than Islamabad.

'Punitive actions'
They said the decision could strengthen those in the government who argue that Washington is a fickle ally who can't be trusted.

"If you still need the relationship, which clearly the United States does, then it really doesn't make sense to take action at this time because it leaves the United States with less, not more, influence with the Pakistani military," said Maleeha Lodhi, a former Pakistani ambassador to the U.S. "Cooperation cannot be coerced by punitive actions."

Tariq Fatemi, another former Pakistani ambassador to the U.S., said he thought the American strategy to pressure Pakistan was destined for failure.

"I think it is unwise to expect the Pakistanis to buckle under what is a publicly delivered snub," said Fatemi. "It will strengthen those elements in the armed forces that have always had grave misgivings of the relationship with the United States."

Former Pakistani President Pervez Musharraf said Monday that the suspension was not in the interest of either nation and could hamper anti-terrorism efforts.

"We are weakening the country and the army," Musharraf said during an address at Rice University's Baker Institute of Public Policy. "It will have a negative effect certainly on the Pakistan army, on its capability to fight terrorism."

The move also stoked anger among ordinary Pakistanis.

"Our country threw the whole country into the inferno of the war on terror and made poverty our destiny, but it could not appease the Americans," said Mohammad Nauman, a 41-year-old information technology specialist in the southern port of Karachi.