|The US military has left a Pakistani airbase as tensions with Islamabad continue to remain high [AFP/ISPR HANDOUT]
Pakistan may continue a suspension of NATO supply routes into Afghanistan for several weeks, the country's prime minister has said.
Speaking to the BBC, Yousuf Raza Gilani also refused to rule out closing Pakistani airspace to the US military.
Pakistan suspended the passage of NATO supplies on routes that run through the country into Afghanistan in protest against a strike by NATO forces on Pakistani border posts last month that killed 24 soldiers, an attack the government termed "a deliberate act of aggression".
Gilani said that there was a "credibility gap" in the relationship with the US. His government is currently carrying out a review of Pakistan's "terms of engagement" with Washington.
"[The suspension of supplies] has already entered its 17th day. Hundreds, if not thousands, of containers are parked on the borders, whereas many more are now waiting at the Karachi port," Al Jazeera's Kamal Hyder, reporting from Islamabad, said.
"The chief minister of Balochistan province is even warning that he wants all these tankers and containers to go back, because they're coming under attack: they're sitting ducks.
"This is indeed a serious crisis, because most of the aviation fuel which is going into Afghanistan is going through strategic corridors, which both go through Pakistan.
"One is at Chaman, from where the cargo moves to Kandahar, and the other [through Torkham] to Kabul."
On Sunday, armed men killed the driver of a truck carrying NATO supplies and torched his, and six other vehicles, all of which were carrying oil for NATO troops in Afghanistan, police said.
The convoy was attacked while returning to the port city of Karachi from the Afghan border at Chaman.
Police said that "around eight gunmen" approached the convoy, ordered it to stop and started firing on the tankers.
"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," said Inayat Bugti, a local police official.
On Sunday, the US vacated the Shamsi air base in Pakistan's Balochistan province after a 15-day ultimatum given by Islamabad following the NATO air raid.
The withdrawal was completed when the final flight carrying US personnel and equipment flew out.
Nine planes carrying personnel and 20 carrying equipment - including drone aircraft and weapons - left for neighbouring Afghanistan, officials said.
Shamsi was believed to be the staging post for US drones operating in northwest Pakistan and neighbouring Afghanistan.
Meanwhile, a senior Pakistani military official told a US television network on Sunday that his country "will shoot down any US drone that enters its territory".
Al Jazeera’s Hyder reported that Pakistani parliamentarians had "asked their air chief whether they had the capability to shoot down the drones if they were to violate their frontiers".
"The air chief told them point blank that the decision to shoot down the drones was a political one and if they were ordered to do so they would comply," he said.
Al Jazeera's Islamabad bureau reports that the Pakistani military has confirmed that border posts have been beefed up with air defence weapons systems to thwart any intrusion or offensive action from across the border.
The decision regarding the Shamsi base was taken last month after PM Gilani called an emergency meeting of his military chiefs.
At the time, the foreign ministry also summoned Cameron Munter, the US ambassador in Islamabad, to "lodge a strong protest" against the attack.
Islamabad has remained adamant over the closure of the air base despite the Obama administration’s pledge to conduct a full investigation into the attack.
In a telephone conversation with US Secretary of State Hillary Clinton on November 27, Hina Rabbani Khar, Pakistan's foreign minister, said such attacks were "totally unacceptable".
"They demonstrate complete disregard for international law and human life, and are in stark violation of Pakistani sovereignty," she said.