| Three sets of attacks have left the future of the key Nato supply route through Pakistan in question [Reuters]
Armed men have killed at least three people and set fire to 20 oil tankers in Pakistan that were en route to Nato and US troops in Afghanistan, according to police.
The attack early on Monday close to the capital Islamabad was the third such strike on supply lorries since Friday.
One of the main routes used by Afghanistan-bound trucks has been closed for days after three Pakistani troops were killed in a helicopter strike by the military alliance in a border area.
Police said the attackers opened fire on trucks that were parked at a poorly-guarded terminal before setting them on fire.
Umer Hayat, a police officer, said three people were killed and blamed Monday's attack on "terrorists". Eight other people were wounded in the incident near the Pakistani capital.
Mohammad Ilyas, the doctor in charge of emergency care in Rawalpindi civil hospital, said: "We received three dead bodies and seven wounded.
"They all had bullet wounds. Two of them were in serious condition but they are improving and we hope they will be in a stable condition soon," Ilyas added.
Omar Hayat, the Islamabad police chief, 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," Hayat said.
Al Jazeera's Kamal Hyder, reporting from Islamabad, said that the blocked supply route is used to move aviation fuel to Nato aircraft in Afghanistan.
"It's that aviation fuel that's used in combat missions inside Afghanistan," he said.
On Friday, up to 30 tankers carrying supplies for Nato troops in Afghanistan were set ablaze in two attacks by unidentified assailants in Shikarpur, in Sindh province.
The Pakistani Taliban claimed responsibility for at least one of them, and vowed to launch more.
No deaths were reported in those attacks.
Striking now gains them more media attention than normal and adds to unease between Pakistan and the United States.
On Thursday, three Pakistani soldiers were killed in cross-border strikes as Nato forces chased anti-government fighters in Pakistan's northwestern Kurram region.
Although there are alternative routes available, Nato troops in Afghanistan rely heavily on the supplies transported from Pakistan through the Khyber Pass.
Coalition officials say recent attacks on supply convoys do not result in shortages in Afghanistan. Hundreds of trucks cross into the country each day.
Source: Al Jazeera and agencies