An outraged Pakistan today slammed 'senseless' NATO air strike on its borders with Afghanistan that left 24 soldiers dead and asked the US to vacate its Shamsi air base within 15 days,saying the incident forces it to revisit the terms of engagement with Washington. In a bid to salvage its already strained ties with Pakistan,the US quickly promised to back a probe into the deadly attack that took place in restive Mohmand tribal region in Khyber-Pakhtunkhawa province yesterday. In Brussels,NATO came out with a regret in the evening,saying the incident was "unintended". Pakistan's anger was conveyed to US 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 US 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 US 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," the statement said. Pakistan-US ties,strained by the May 2 American covert raid that killed Osama bin Laden in Abbottabad,plunged to a new low after the incident.