Despite being a frontline partner in the war on terror, Pakistan remains a major source of Islamic extremism and the home for some top terrorist leaders, a report has said.

The Congressional Research Service in its latest report identified Al Qaeda as the single greatest terrorist threat to the United States and its interests, and warned that the organisation's core elements maintain active connections and relationships that radiate outward from their leaders' secure hideouts in Pakistan.

"The US also remains concerned with indigenous extremist groups in Pakistan, and with the ongoing cross-border infiltration of Islamist militants who traverse the Line of Control and other borders to engage in terrorist acts in India. Many analysts consider such activities conceptually inseparable from the problem of Islamist militancy in western Pakistan and in Afghanistan," the CRS report, authored by US foreign affairs specialist Alan Kronstadt, said.

He said many experts rejected efforts by the Pakistani government and others to draw significant distinctions between the US and Indian-designated terrorist groups fighting in Jammu and Kashmir and those fighting in western Pakistan and Afghanistan and in Pakistan's interior.

The US reportedly has received pledges from Islamabad that all cross-border terrorism would cease and that any terrorist facilities in Pakistani-controlled areas would be closed. Similar pledges have been made to India, Kronstadt, said adding that numerous experts have raised questions about the 'determination, sincerity, and effectiveness of Pakistani government's efforts to combat religious extremists.'