WASHINGTON -- The Obama administration is withholding assistance to some Pakistani military units over concerns they may have been involved in human rights abuses, including extrajudicial killings and torture, a senior U.S. official said Thursday.

The official said aid to a handful of Pakistani units believed to have committed, encouraged or tolerated abuses had been suspended under 1997 legislation championed by Sen. Patrick Leahy, D-Vt. The so-called Leahy Amendment bars U.S. military assistance from going to foreign armed forces suspected of committing atrocities.

"In accordance with the Leahy Amendment, we have withheld assistance from a small number of units linked to gross human rights violations," the official said. "At the same time, we have encouraged Pakistan to improve its human rights training, and it is taking steps in that direction."

The official spoke on condition of anonymity because he was not authorized to speak publicly on the matter.

It was not immediately clear when the decision to withhold the assistance was first taken or exactly how many Pakistani military units were affected. The New York Times first reported the withholding of aid late Thursday.

Halting assistance to certain units will not affect broader U.S. support for Pakistan's military, which is considered key to counterterrorism efforts in the region.

The U.S. is expected on Friday to announce a new military aid package for Pakistan worth about $2 billion over the next five years as it presses the country to do more to fight extremists there and in neighboring Afghanistan.

That announcement is planned at the end of three days of high-level strategic talks in Washington between top U.S. and Pakistani officials, including Pakistani army chief Gen. Ashfaq Parvez Kayani. Kayani has pledged to improve the Pakistani military's human rights record in numerous discussions with U.S. officials.

"As General Kayani has said repeatedly in public and in private, professional standards and enforcement of those standards are the hallmark of a modern and successful military," White House spokesman Tommy Vietor said in a statement. "These issues are part of our conversations with all militaries around the world with whom we work."