A top US commander has said extremists pose the most serious challenge to the "very existence" of Pakistan, but many of its leaders still believe that India is the primary threat and some even regard such groups as a potential strategic asset against New Delhi.

"The Pakistan state face a rising - indeed, an existential - threat from extremists such as al-Qaeda and other transnational terrorist organisations, which have developed in safe havens and support bases in ungoverned spaces in the Afghanistan-Pakistan border regions," US Central Command (CENTCOM) chief General David Petraeus said on Wednesday.

"Nevertheless, many Pakistani leaders remain focused on India as Pakistan's principal threat, and some may even continue to regard Islamist extremist groups as a potential strategic asset against India," he said in a testimony before Senate Armed Services Committee on Afghanistan and Pakistan.

Meanwhile, al-Qaeda and the Taliban and other insurgent groups operating from the border region are engaged in an increasingly violent campaign against Afghan and coalition forces and the developing Afghan state, he said.

While he did not identify any country, General Petraeus said: "Some countries in the (CENTCOM) AOR (area of operation) play a dangerous game of allowing or accepting extremist networks and terrorist facilitators to operate from or through their territory, believing that their own people and governments will be immune from the threat."

General Petraeus said the US would continue to target, disrupt and pursue the leadership bases and support networks of al-Qaeda and other transnational extremist groups operating in the region.

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