US airstrikes that may have killed Pakistani soldiers along the Afghan border was a legitimate act of self-defence, the Pentagon said on Wednesday.
US forces called in the air raid after coalition troops came under fire from Pakistan, Pentagon spokesman Geoff Morrell said.
The attack, according to the Pakistani authorities, left 11 of their soldiers dead and sparked a rupture between the two counties and close allies in the war on terrorism.
"Every indication we have is that this was a legitimate strike against forces that had attacked members of the coalition," Morrell said.
"US forces, operating on the border of Pakistan in Afghanistan territory, came under attack from hostile forces, and - in self-defense - they called in an airstrike, which took out those forces that were attacking them."
Morrell could not confirm that Pakistani soldiers were among the dead, but the US State Department, whose ambassador was summoned by the Pakistani Foreign Ministry, expressed regret over the incident.
"This is a regrettable incident. We're sad to see the loss of life among the Pakistani military, who are partners in fighting terror," Gonzalo Gallegos, a US State Department spokesman, said.
The military said it bombed a post 200 metres deep into Pakistan where insurgents were firing on the US patrol inside Afghanistan. US forces never crossed the border, Morrell said.
The Pakistani government heavily criticized the US over the incident that could complicate already difficult relations as a new parliament takes hold.
Gallegos said the incident shows there is a need to improve communication between US-led forces in Afghanistan and Pakistani authorities to ensure similar episodes do not take place in the future.
"This is a reminder that better cross-border communications between forces is vital," Gallegos said.
The coalition aircraft destroyed the paramilitary Frontier Corps post in the Mohmand tribal region used as refuge by Taliban militants, the Pakistani Army said.
The strike was a "completely unprovoked and cowardly act," the Army said in a statement adding that it "hit at the very basis of cooperation and sacrifice with which Pakistani soldiers are supporting the coalition in war against terror."
The US ambassador, Anne Patterson, was called to the Foreign Ministry, but Washington did not provide information about the meeting.
The US military said three aircraft dropped more than a dozen bombs after a two-hour firefight. There were no reports of US casualties.
"At no time did coalition ground forces cross into Pakistan," the US military said in a statement, adding that an investigation into the exchange has been launched.
Some media reports in Pakistan put the casualty figure for the Pakistani soldiers at 13.
Pakistan has deployed some 100,000 troops along its border with Afghanistan to contain cross-border attacks by Al Qaeda and Taliban militants, who have safe havens in the country's tribal areas.
US forces have occasionally targeted militants' hideouts on Pakistani soil, mostly by using pilotless aircraft.
The new coalition government in Islamabad has started peace talks with the local Taliban, a move that has been greeted sceptically by Washington.
Maulvi Omar, a spokesman for Pakistani Taliban, claimed the allied planes' bombardment started after more than 60 "mujahidin" attacked Afghan forces trying to enter Pakistani territory on Tuesday night.