A prominent U.S. newspaper says high-level talks designed to end a diplomatic deadlock between the U.S. and Pakistan have ended in failure because Pakistan has demanded an unconditional apology from Washington for an airstrike.

The New York Times says Marc Grossman, the U.S. special envoy to Afghanistan and Pakistan, left Islamabad Friday night after two days of discussion. The talks failed to yield a remedy to the damage caused by a U.S. airstrike in November that killed 24 Pakistani soldiers on the Afghanistan border.

The U.S. has refused to apologize for the strike and Pakistan has retaliated by cutting off NATO supply routes to Afghanistan. The two countries disagree about the sequence of events in the deadliest single cross-border attack of the 10-year war in Afghanistan.

The Times say the U.S. was “seriously debating” whether to say “I'm sorry” to Pakistan, until April 15, when multiple, simultaneous attacks struck Kabul and other Afghan cities. U.S. military and intelligence officials say the attacks came from the Haqqani network, a terrorist group based in Pakistan's tribal belt in North Waziristan.

The New York Times said the attacks “swung the raging debate” on whether U.S. President Barack Obama or another senior U.S. official should go beyond the expression of regret Washington has already given, or apologize.

Pakistani officials say they cannot re-open the NATO supply routes into Afghanistan without an apology. In return, the U.S. is withholding as much as $3 billion of promised military aid.

The newspaper says the continuing deadlock “does not bode well” for Pakistan's attendance at a NATO meeting in Chicago in three weeks, assuming it is even invited.

The New York Times reports U.S. administration officials said Friday the stalemate would not be resolved quickly.