"The problem is that there are forces within the Pakistani government -- specifically in Pakistan's Inter- Services Intelligence or ISI -- that refuse to take similar steps against all the terrorist groups active in Pakistan, tolerating some groups -- or even worse," Adam Szubin, Acting Under Secretary on Countering the Financing of Terrorism, told a Washington audience.
In his remarks at the Paul H Nitze School of Advanced International Studies, Szubin said at the same time, Pakistan has been -- and remains -- a critical counterterrorism partner in many respects.
"Of course, Pakistanis are themselves often the victims of brutal terrorist attacks on schools, markets, and mosques, and the list unfortunately goes on. And in the face of such violence, Pakistan has in some ways pushed back," Szubin said.
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But the ISI problem of supporting terrorist groups continues, he said.
"This is a distinction we cannot stand for," Szubin asserted.
The US has been saying that Pakistan has not yet placed adequate pressure on the Haqqani Network of militants to prevent them from plotting deadly cross-border attacks in war- torn Afghanistan.
Afghan authorities allege leaders of the Haqqani group, which is fighting alongside the Taliban, are directing high- profile attacks, particularly in the capital, Kabul, from their sanctuaries on Pakistani soil, with the covert support of the country's intelligence operatives.
"The Haqqanis operationally have been able to continue to conduct operations inside Afghanistan. They constitute the primary threat to Americans, to coalition members and to Afghans, especially in and around Kabul," he said.
Pakistani authorities routinely deny the presence of any sanctuaries and insist counter-terrorism military operations have targeted and uprooted all militant infrastructures on their side of the border.