ISLAMABAD – Pakistan’s main spy agency continues to arm and train the Taliban and is even represented on the group’s leadership council despite U.S. pressure to sever ties and billions in aid to combat the militants, said a research report released Sunday.

The findings could heighten tension between the two countries and raise further questions about U.S. success in Afghanistan since Pakistani cooperation is seen as key to defeating the Taliban, which seized power in Kabul in the 1990s with Islamabad’s support.

U.S. officials have suggested in the past that current or former members of Pakistan’s powerful Inter-Services Intelligence agency, or ISI, have maintained links to the Taliban despite the government’s decision to denounce the group in 2001 under U.S. pressure.

But the report issued Sunday by the London School of Economics offered one of the strongest cases that assistance to the group is official ISI policy, and extends to the highest levels of the Pakistani government.

“Pakistan’s apparent involvement in a double-game of this scale could have major geopolitical implications and could even provoke U.S. countermeasures,” said the report, which was based on interviews with Taliban commanders, former Taliban officials, Western diplomats and many others.

“Without a change in Pakistani behavior it will be difficult, if not impossible, for international forces and the Afghan government to make progress against the insurgency,” said the report, by Matt Waldman, a fellow at Harvard University’s John F. Kennedy School of Government.