The US military has admitted it bears significant responsibility for last month's air strike on the Afghan border that killed 24 Pakistani soldiers. A statement said US and Afghan forces acted in self defence, but conceded there had been a lack of proper co-ordination with Pakistani forces. BBC correspondents say the admission is expected to embarrass the US military. In retaliation for the killings, Pakistan closed its border with Afghanistan, cutting Nato supply lines. There was no immediate response from Pakistan to the findings of the US investigation. Pakistan, a vital partner in the fight against militants in the region, has demanded a formal US apology. In the statement the US once again expressed its deepest regret for the "tragic loss of life" caused by the air strike in Mohmand tribal agency on 26 November. "Inadequate co-ordination by US and Pakistani military officers operating through the border co-ordination centre - including our reliance on incorrect mapping information shared with the Pakistani liaison officer - resulted in a misunderstanding about the true location of Pakistani military units," it said. "This, coupled with other gaps in information about the activities and placement of units from both sides, contributed to the tragic result."