QUETTA, Pakistan - Assailants torched more than 20 tankers in Pakistan carrying fuel for U.S. and NATO troops in neighboring Afghanistan on Thursday, in the first reported attack since Islamabad closed the border to protest coalition air strikes that killed 24 Pakistani troops last month.

Several hundred trucks have been stranded at poorly guarded terminals around the country as they wait for Pakistan to reopen its two border crossings into Afghanistan. About 40 percent of the nonlethal supplies for U.S.-led troops in landlocked Afghanistan travel across Pakistani soil.

Islamabad closed both frontier crossings into Afghanistan on Nov. 26, hours after air strikes by the U.S.-led coalition killed 24 troops on the Pakistan-Afghanistan border. U.S. and NATO officials have said the incident was a mistake and have pledged to investigate.

Police officer Hamid Shakil said that unknown men fired rockets at a terminal for the tankers close to the southwestern city of Quetta. He said that at least 23 tankers were set ablaze. There were no immediate reports of casualties.

Last year, Pakistan temporarily closed one of its Afghan crossings to NATO supplies after U.S. helicopters accidentally killed two Pakistani soldiers. Suspected extremists or criminals took advantage of the impasse to launch many attacks against stranded or rerouted trucks carrying NATO supplies.

The deadly air strikes at the border sent tense relations between Pakistan and the United States to new lows, threatening Islamabad's cooperation in helping negotiate an end to the Afghan war.

It came amid political tensions in Islamabad following the resignation of Pakistan's ambassador to the United States after an outcry from the country's powerful military establishment, which is in charge of Afghan and U.S. policy. Envoy Husain Haqqani stepped down because of allegations that he wrote a memo to Washington asking for its help to stop a supposed military coup.

President Asif Ali Zardari has been under pressure because of the scandal and on Tuesday flew to Dubai for medical treatment related to a heart condition. His trip led to rumors that the 56-year-old was losing his grip on power.

Earlier Thursday, Secretary of State Hillary Rodham Clinton said the United States expected that Zardari, an American ally, "will be able to return in full health in his duties" after receiving treatment. A statement for the presidency said Zardari's health was improving.