Washington:  As its ties with Pakistan plunged into a deep crisis after the killing of 28 Pakistani soldiers in a NATO strike along Pak-Afghan border, the US has said it is closely monitoring reports about the cross-border incident and fully backs an investigation.

Secretary of State Hillary Clinton and Defence Secretary Leon Panetta "have been closely monitoring reports of the cross-border incident in Pakistan", the US Departments of State and Defence said in a joint statement.

"Both offer their deepest condolences for the loss of life and support fully NATO's intention to investigate (Saturday’s incident) immediately," the statement said.

Following the cross-border strike on two Pakistani border posts by NATO helicopters and combat jets, Islamabad shut down the crucial NATO supply line and asked the US to close its secret Shamsi air based inside the country which is reportedly used by the CIA for drone strikes.

Clinton, Chairman of the Joint Chiefs of Staff Gen Martin Dempsey and Commander of US and NATO forces in Afghanistan Gen John Allen also called their Pakistani counterparts following the incident.

"Secretary Clinton, Gen Dempsey and Gen Allen each called their Pakistani counterparts as well. (US) Ambassador (to Islamabad Cameron) Munter also met with Pakistani government officials in Islamabad.

"In their contacts, these US diplomatic and military leaders each stressed -- in addition to their sympathies and commitment to review the circumstances of the incident – the importance of the US-Pakistani partnership, which serves the mutual interests of our people," the joint statement said.

All these leaders "pledged to remain in close contact with their Pakistani counterparts going forward as we work through this challenging time," it said.

 Meanwhile, analysts here said the incident is expected to further deepen the rift between the two countries.

The bilateral ties are already under severe strain following the May 2 American raid that killed Osama bin Laden in Abbottabad.

"How big the rift now is between Washington and Islamabad, and to what extent it can be patched, remains to be seen, but the incident appears unprecedented and Pakistan has already demonstrated a willingness to issue a quick and serious response," said Stratfor, a Texas-based intelligence analyst organisation.

Pakistan Prime Minister Yousuf Raza Gilani last night chaired an emergency meeting of Defence Committee of the Cabinet (DCC) comprising his country's top Generals and national security team to take stock of the situation.