Announcement follows official reprimand from Islamabad over recent increase in attacks by remote-controlled aircraft

Al-Qaida's second in command was killed in a drone strike in Pakistan this week, US officials have said, hours after Pakistan officially reprimanded a top US diplomat, declaring such attacks to be against international law and in violation of its sovereignty.

White House spokesman Jay Carney said the death of Abu Yahya al-Libi was a major blow to al-Qaida, and that it would be hard for the group to find someone of similar stature to replace him.

"His death is part of the degradation that has been taking place to core al-Qaida during the past several years and that degradation has depleted the ranks to such an extent that there's no clear successor," Carney said.

"He was among al-Qaida's most experienced and versatile leaders," another US official said.

Letters captured during the US raid in which the former al-Qaida leader Osama bin Laden was killed last year show Libi to have been one of a handful of al-Qaida officials Bin Laden relied on to argue al-Qaida's case to a worldwide audience of militants, particularly the young.

A Pakistani intelligence official also said Libi was dead, but declined to say how authorities knew this or whether they had seen his body.

A Pakistani Taliban leader confirmed Libi's death to Reuters, saying it was a "big loss". Asking not to be named, he said: "After Doctor sahib [al-Qaida leader Ayman al-Zawahri], he was the main al-Qaida leader."

Earlier Richard Hoagland, the US chargé d'affaires in Islamabad, was called into the foreign ministry on Tuesday after a recent increase in missile attacks by remotely controlled aircraft. He was told "drone strikes represented a clear red-line for Pakistan", a government statement said. The diplomatic step comes with Islamabad looking ever more isolated in the region as the US seeks to reduce its dependence on Pakistan.

The timing of Pakistan's stand has highlighted the immense stresses the drone campaign is putting on efforts by Washington and Islamabad to repair their deeply distrustfulrelationship.

Pakistani officials had said Libi, who once escaped from a top security US military prison, might have been killed by a drone strike on Monday on Hasokhel, a village in North Waziristan, where Taliban and al-Qaida militancy thrives. The attack was reported to have claimed up to 18 lives.

Libi enjoyed legendary status within the movement. His death underlines the importance the Obama administration places on drones in its fight against terrorists.

Al-Qaida can often wait weeks or even months to admit to the deaths of its senior commanders.

Syed Amid, a tribal elder from the area, said he was not aware whether Libi was killed, but said up to 18 militants had been killed and "some foreigners were among the dead", including Arabs.

A Pakistani intelligence official said he believed Libi was in the house that was targeted by missiles and that one of his vehicles was also destroyed.

Although drone strikes are extremely unpopular within Pakistan, some people living in the semi-autonomous tribal areas where the strikes are concentrated say they support them.

Nazim Dawar, a social worker from the town of Mir Ali, said there were never any popular demonstrations in his area against drones. "It is only the people in the cities like Islamabad, Karachi and Peshawar that agitate against them," he said.

"Yes, some ordinary people do get killed, but it is mostly the Taliban who get killed in these attacks."

But Mohammad Iqbal, a labourer from North Waziristan, said the strikes were "pulling apart the social and economic fabric" of the tribal areas.

"About half the people have had to move to other areas to escape the drones," he said. "Anyone who stays lives in terror they will be killed." In Pakistan's view, the attacks have to stop. As the country's foreign ministry pointed out to Hoagland, the country's parliament had "emphatically stated [drone strikes] were unacceptable".

"He was informed that the drone strikes were unlawful, against international law and a violation of Pakistan's sovereignty," a government statement said.

Earlier this year Pakistan's parliament unanimously decided on a set of demands the government should make of the US. They included an end to drone attacks, higher tariffs for Nato supply trucks crossing Pakistani territory and a public apology for a disastrous incident in November when US troops killed 24 Pakistani soldiers. The accident prompted Pakistan to close its borders to trucks carrying vital military supplies to US and Nato forces in Afghanistan. The closure has still not been overturned, despite several false hopes.

US diplomats are adamant there will be no ending of the drone campaign. The best Pakistan can hope for, they say, is some sharing of intelligence and inclusion in target selection. And they are furious with Pakistan's behaviour, not least during the recent Nato conference in Chicago, where it had been hoped Islamabad would announce it would restart co-operation on supply lines and counter terrorism.

Drone strikes, which had been in abeyance, restarted with a vengeance after the failure of the Chicago summit, with three attacks in three days starting on Saturday, that have been reported to have killed up to 29 people.

Many in Pakistan's security establishment believed the US-led war in Afghanistan would grind to a halt without the vital supply lines from the port of Karachi and then overland to Kabul and Kandahar.

But that has not come to pass, with the US and its allies instead switching to the far longer and more costly route through Russia and central Asia into northern Afghanistan.On Monday, Nato announced that the former Soviet republics had agreed that in addition to bringing vital supplies into Afghanistan, the alliance would be permitted to move a decade's worth of equipment out through their territory as well.

It had been thought Pakistan's co-operation would be vital for the safe withdrawal of Nato kit.

The Pakistani reprimand came as the US defence secretary, Leon Panetta, was in Delhi to encourage India – Pakistan's arch enemy – to become more militarily involved in Afghanistan, including doing more to train Afghan soldiers.

For decades much of Pakistan's foreign policy has been geared to reducing Indian influence in Afghanistan out of fear it would face a two-flank struggle if it ever got into another military confrontation with a traditional rival it has already fought three wars against.

"Before the US saw Pakistan as the best partner to stabilise Afghanistan and India was being told by the US 'don't do too much' [in Afghanistan], don't upset the Pakistanis'. Now that has changed," said Dr C Raja Mohan, at Delhi's Observer Research Foundation. The local Times of India newspaper reported on Tuesday that India is now close to signing huge deals for artillery and helicopter gunships worth more than $2bn from the US.