Gunmen attacked Nato oil tankers stranded in southwest Pakistan for the second time in days as Islamabad warned it could enforce its blockade of the US lifeline into Afghanistan for weeks.
The attackers shot dead a driver and destroyed seven tankers in a blaze of fire late Sunday, the second attack in four days in Pakistan's volatile region of Baluchistan, which is rife with separatists and Taliban militants.
There was no claim of responsibility but Pakistan's fragile alliance with the United States crashed to new lows after November 26, when Nato air strikes killed 24 Pakistani soldiers in what Pakistan said was a deliberate attack.
Islamabad sealed its Afghan border to Nato convoys, closures that entered a 17th day on Monday, forcing trucks back to the Arabian Sea port of Karachi.
Sunday's convoy was targeted in Dadar town, 90 kilometres (56 miles) southeast of Quetta, the capital of Baluchistan, police said.
"Around eight gunmen approached the convoy on motorcycles in Bolan district, ordered it to stop and started firing on the tankers," senior local police official Inayat Bugti told AFP.
"A driver of one of the tankers was also hit by a bullet and was killed instantly. The attackers later put the tankers on fire and escaped," he said.
Last Thursday, gunmen destroyed at least 34 trucks in a gun and rocket attack at a temporary Nato trucking terminal in Quetta.
The Taliban have in the past said they carried out such attacks to disrupt supplies for the 140,000 US-led international troops fighting in Afghanistan.
Prime Minister Yousuf Raza Gilani told the BBC that Pakistan's blockade of the border, already the longest since the 2001 US-led invasion of Afghanistan, will not be lifted until new "rules of engagement" were agreed with Washington.
"We are working together and still we don't trust each other. I think we have to improve our relationship," he said in the interview aired Sunday.
"We want to set new rules of engagement and cooperation with United States. We have a resolve to fight against terrorism and therefore we want to set new rules of engagement."
Despite US commanders' insistence that the attack was not deliberate, Gilani stood by Pakistan's position that it was pre-planned.
US President Barack Obama telephoned President Asif Ali Zardari to offer his condolences over the strike, but Washington has stopped short of apologising pending the outcome of a military probe due out on December 23.
On Sunday, Pakistani officials said US personnel had left the Shamsi air base in Baluchistan, which they were ordered to vacate after the strikes.
The air base was widely reported to have been a hub for a covert CIA drone war targeting Taliban and al Qaeda fighters on Pakistani soil.
Pakistani-US relations have been in freefall this year, battered in January when a CIA contractor shot dead two men in Lahore and in May when US Navy SEALs killed Osama bin Laden near the capital without informing Islamabad first.