Islamabad: An outraged Pakistan slammed "senseless" Nato airstrike on its borders with Afghanistan, which left 24 soldiers dead. The country also asked the U.S. to vacate its Shamsi air base within 15 days, saying the incident forces it to revisit the terms of engagement with Washington.

To salvage its already strained ties with Pakistan, the U.S. quickly promised to back a probe into the deadly attack that took place in restive Mohmand tribal region in Khyber-Pakhtunkhawa province. In Brussels, Nato came out with a regret, saying the incident was "unintended".

Pakistan's anger was conveyed to U.S. Secretary of State Hillary Clinton by foreign minister Hina Rabbani Khar who said the incident "negates the progress made by the two countries on improving relations and forces Pakistan to revisit the terms of engagement".

Following the cross-border strike, Islamabad shut down the crucial Nato supply lines to Afghanistan and asked the U.S. to vacate within 15 days the Shamsi air base inside Pakistan, which is reportedly being used by the CIA for drone strikes.

"The foreign minister conveyed to the Secretary of State the deep sense of rage felt across Pakistan at the senseless loss of 24 soldiers due to the Nato/ISAF attack," said a statement from Pakistan's Foreign Office.

In Washington, a joint statement issued by the U.S. departments of state and defence said that Clinton and defence secretary Leon Panetta "have been closely monitoring reports of the cross-border incident in Pakistan". "Both [Clinton and Panetta] offer their deepest condolences for the loss of life and support fully Nato's intention to investigate immediately."

Nato secretary-general Anders Fogh Rasmussen expressed his regret to PM Yousuf Raza Gilani. "The deaths of Pakistani personnel are as unacceptable and deplorable as the deaths of Afghan and international personnel... This was a tragic unintended incident. I fully support the ISAF probe which is underway," he said in a statement.