WASHINGTON: Pakistan is a "haven" for several Islamist terror groups and successive Pakistani governments are widely believed to have supported some outfits as proxies in the country's historical conflicts with its neighbours, according to a US Congressional research report."Pakistan is a haven for numerous Islamist extremist and terrorist groups, and successive Pakistani governments are widely believed to have tolerated and even supported some of these as proxies in Islamabad 's historical tensions and conflicts with neighbours," said the latest report by the independent Congressional Research Service (CRS) on Pakistan.The report, dated May 14, notes that Pakistan's security services are seen by many independent analysts to be too willing to make distinctions between what they consider to be "good" and "bad" Islamist extremist groups, maintaining supportive relations with Afghan insurgents and anti-India militant groups operating from Pakistani territory.The periodic CRS report on Pakistan is not an official report of the United States Congress , but prepared by noted experts on the issue to keep the lawmakers informed about the current state of affairs in Pakistan, the bilateral relationship and ties with its neighbours.Noting the key steps taken by the Narendra Modi-led government in the last one year, including the decision to invite Prime Minister Nawaz Sharif for his swearing-in ceremony and sending Foreign Secretary S Jaishankar to Islamabad on a SAARC Yatra, the report states that "serious tension" remains between the two South Asian neighbours."Serious tensions between Pakistan and India persist, and many observers see the Pakistani Army obstructing the efforts of Pakistani business interests to deepen commercial trade and other engagement with India, seeking resolution of territorial disputes as a prerequisite," said the CRS report, authored by K Alan Kronstadt , specialist in South Asian Affairs.According to the report, Pakistan's more recent and apparently energetic development of short-range, nuclear-armed missiles -- ostensibly in response to India's purported "cold start" doctrine of rapid preemptive strikes with conventional forces -- has raised fears about negative effects on crisis stability in the event of open warfare between Pakistan and India."The security of Pakistan's nuclear arsenal, materials, and technologies continues to be a top-tier US concern, especially as Islamist militants have expanded their geographic influence there," the report said.Pakistan, it said, is among the leading recipients of US foreign assistance in the post-9/11 period, with Congress appropriating more than USD 18 billion in such assistance during 2002-2015. This includes USD 10.5 billion in economic, development and humanitarian aid, and over USD 7.6 billion in security-related aid, it said.