ISLAMABAD, Pakistan - Pakistan kept a key border crossing into Afghanistan closed to NATO convoys Thursday, despite U.S. apologies for a fatal airstrike that killed or wounded six Pakistani soldiers and prompted the blockade.

In a news conference in this capital, Foreign Ministry spokesman Abdul Basit said Pakistan was still considering whether to reopen the Torkham pass in the northwestern part of the country.

"Our authorities are evaluating the security situation, and a decision regarding the reopening of the supply route will be taken in due course," he said.

As trucks carrying fuel and supplies for NATO troops remained idle at the crossing, a new apology was issued by Adm. Mike Mullen, chairman of the Joint Chiefs of Staff. In a letter written to Gen. Ashfaq Parvez Kiyani and released Thursday by the U.S. Embassy here, Mullen said U.S. military officials would "review the investigation thoroughly with an eye toward avoiding recurrence of a tragedy like this."

That followed two other U.S. apologies issued Wednesday, nearly a week after the U.S. missile strike, acknowledging that two of the American military's Afghanistan-based assault helicopters had entered Pakistani airspace "several times" and mistakenly fired at a military post.

Statements from the U.S. Embassy in Islamabad and coalition force headquarters in Kabul largely agreed with Pakistan's initial assessment that its troops had fired rifle shots to warn the helicopters they were on the Pakistani side of the border. The helicopters, on an anti-insurgent mission, responded with missiles that destroyed the post, killing two Pakistanis and wounding four. Coalition statements initially said the missiles were fired in self-defense.

"We deeply regret this tragic loss of life and will continue to work with the Pakistan military and government to ensure this doesn't happen again," Gen. David H. Petraeus, the coalition commander in Afghanistan, said in a military statement that pledged better coordination. U.S. Ambassador Anne W. Patterson extended "our deepest apology to Pakistan and the families" of the casualties.

A senior Pakistani military official described the statements as "good gestures" that would be "taken positively by everybody in Pakistan," along with an assurance that "these attacks won't be repeated." Pakistan had demanded a statement of fault and an apology.

The main Torkham border crossing between Pakistan and Afghanistan, shut in response to the attack, remained blocked Wednesday, extending a backup of coalition military supplies. Gunmen have carried out several attacks on supply trucks in Pakistan, including a strike Wednesday in which up to 25 fuel tankers were torched in the southwestern city of Quetta.

The fatal airstrike, the latest in a series of air incursions into Pakistan, heightened tensions between the South Asian nation and the United States, uneasy allies even in the best of times. The U.S.-led war in Afghanistan is very unpopular in Pakistan, where many see it as a catalyst for homegrown militancy.

The Obama administration has become frustrated with Pakistani reluctance to launch a full offensive against insurgent sanctuaries in the border region, from which attacks in Afghanistan are launched. Beginning late last year, the administration has issued several warnings to Pakistan that if it does not move aggressively against the sanctuaries, the United States will have to take action.

A report President Obama sent to Congress this week criticized Pakistani efforts. "The Pakistan military continued to avoid military engagements that would put it in direct conflict with Afghan Taliban or al-Qaeda forces in North Waziristan," the remote border area where sanctuaries are located, it said. "This is as much a political choice as it is a reflection of an under-resourced military prioritizing its targets."