Gen Martin Dempsey, the most senior figure in the US military has refused to apologise for an air strike at the weekend that killed 24 Pakistan soldiers.
Gen Dempsey, the Chairman of the Joint Chiefs of Staff, spoke as Pakistan closed its borders to Nato convoys supplying international troops in Afghanistan.
The Pakistani army claimed the attack lasted almost two hours, and that it continued even after commanders on the ground contacted Nato to ask what was going on.
In an interview with ITV News, Gen Dempsey said he telephoned the Chief of Pakistan’s army General Ashfaq Kayani and “expressed regret”
“They have reason to be furious that they have 24 soldiers dead and that what killed them was the ordinance of a partner,” he said.
Asked if there was anything apologise for however, Gen Dempsey said: “absolutely not”.
Gen Dempsey also admitted that the US relationship with Pakistan, viewed from the outside is “the worst it’s ever been”.
The White House last night said President Barack Obama was taking the incident “very seriously” and wanted to get to the bottom of the “tragedy”.
Closing the crossings will halt almost half of supplies for the Nato-led force, including British troops.
Accounts differ about what happened in the early hours of Saturday when American aircraft attacked two border posts inside Pakistan, but the fallout is clear: a deep diplomatic crisis threatening cooperation against Taliban and al-Qaeda militants.
Pakistan immediately shut its borders to convoys carrying fuel and supplies and said it was reviewing all military and diplomatic ties with the US and Nato.
Rehman Malik, Pakistan’s interior minister, said: “Nato forces should respect the feelings of the Pakistani nation.” He added that trucks and tankers already in the country would not be held back.
Although the US is transporting more of its equipment, food and fuel through central Asia in an attempt to reduce Pakistan’s leverage, the route through Karachi still accounts for 49 per cent of supplies destined for the 140,000-strong foreign force.
The International Security Assistance Force have played down the impact of closing the road. Lieutenant Gregory Keeley, a spokesman in Kabul, said: “ISAF uses a vast supply and distribution network to ensure coalition forces remain well-stocked in order to carry out their assigned mission across Afghanistan.”
Both sides blamed the other for the air strike. Western and Afghan officials say the raid was a response to firing from the Pakistani side of the border.
Pakistani soldiers wounded in the attack have told The Daily Telegraph there was no militant activity in the area.
Maj Gen Athar Abbas, an army spokesman, said the troops were victims of unprovoked aggression, and that commanders urged Nato to “get this fire to cease”. “Somehow it continued,” he said.