MIRANSHAH, Pakistan (AFP) – Pakistani Taliban militants Monday claimed two fiery attacks on Afghanistan-bound NATO supply convoys in which nearly 60 trucks have been torched in three days -- and vowed more to come.

At least three people were killed when 20 NATO oil tankers were set ablaze by attackers armed with Molotov cocktails near the Pakistani capital, as a separate blockade of a main land route for NATO supplies continued for a fifth day.

Television pictures showed towers of flame springing from the trucks, which were filling up just outside Islamabad en route to Afghanistan early in the morning.

In a similar incident on Friday in the south, heavily armed gunmen set ablaze more than two dozen trucks and tankers carrying fuel for the 152,000-strong foreign forces fighting the Taliban-led insurgency.

Police said around a dozen people who attacked the supply tankers fled the scene.

Both attacks were claimed by Pakistani Taliban on Monday.

"We accept responsibility for the attacks on the NATO supply trucks and tankers," Tehreek-e-Taliban Pakistan (TTP) spokesman Azam Tariq told AFP.

"I am talking about attacks both in Sindh and in Islamabad," he said in a telephone call from an undisclosed location.

"We will carry out more such attacks in future. We will not allow the use of Pakistani soil as a supply route for NATO troops based in Afghanistan.

"This is also to avenge drone attacks," he added.

The United States has recently ramped up its drone campaign in Pakistan's lawless northwest tribal region on the Afghan border, which it calls the global headquarters of Al-Qaeda and is a hub of militants fighting in Afghanistan.

Ambushes of NATO convoys are not uncommon, but are normally concentrated in the militant strongholds of the northwest, where Pakistan has closed a key land crossing into Afghanistan after a cross-border NATO attack.

Police officials confirmed the death toll and said the tankers were attacked as they were parked up at the Attock oil refinery outside the capital for refuelling.

"As they were waiting to get the oil, some people opened fire and threw molotov cocktails at the tankers. The security guards retaliated and the gunfire continued for some time," said Islamabad police chief Omar Hayat.

Queues of more than 200 trucks and oil tankers have formed at the border in the northwest tribal area of Khyber as they wait to deliver supplies.

Pakistan's ambassador to the United States before the assault on Sunday said the closed transit route would reopen "relatively quickly".

Pakistan blocked the crossing on Thursday after a NATO helicopter strike that Islamabad says killed three of its soldiers. The alliance said it shot back in self-defence.

After a flurry of phone calls and pressure from ally the United States, Hussain Haqqani, Pakistan's ambassador to Washington, told CNN's "State of the Union" programme that the crossing would reopen in "less than a week".

"I think the supply line will be open relatively quickly," he said.

The Khyber pass at Torkham is on one of the key NATO supply routes through Pakistan into war-torn Afghanistan.

The cross-border raid was the fourth in a week by NATO helicopters pursuing militants into Pakistan, which condemned the action as a serious breach of its sovereignty, threatening to destabilise ties with backer Washington.

Pakistani security official said a two-member Pakistan team has joined a joint investigation into the incident by the NATO-led International Security Assistance Force (ISAF) and US officials.

The envoy Haqqani said that he had received a phone call from General David Petraeus, the US commander of NATO forces in Afghanistan.

"He understands Pakistan has not stopped it as a political retaliation but only to make convoys more secure," Haqqani said, adding the issue was unlikely to cause any permanent damage to future US-Pakistan cooperation.