High level US-Pak talks to break a diplomatic deadlock have ended in failure after Washington refused to tender an apology to Pakistan over a recent cross-border raid as it was angered by new Taliban attacks on Afghan capital Kabul.

Marc Grossman, the US special envoy to Afghanistan and Pakistan left Islamabad on Friday with no agreement after two days of discussions aimed at patching up the damage caused by American airstrikes last November that killed 24 Pakistani soldiers, the New York Times reported.

Pakistan had demanded an unconditional apology from the US over the incident, but the talks ended in failure, as the White House, "angered by the recent spectacular Taliban attacks in Afghanistan, refuses to apologise," it said.

The US and Pakistan insist that they are ready to patch up and restore an uneasy alliance that at its best offers support for American efforts in Afghanistan as well as the battle against some extremist groups operating from Pakistan, it said.

The administration had been seriously debating whether to say 'sorry' to the Pakistanis' satisfaction.

However things changed on April 15 when multiple, simultaneous attacks struck Kabul and other Afghan cities.

"What changed was the 15th of April," said a senior administration official in the report. American military and intelligence officials concluded the attacks came at the direction of Haqqani network, a group working from North Waziristan in Pakistan's tribal belt.

"That confirmed longstanding American mistrust about Pakistani intentions — a poison that infects nearly every other aspect of the strained relationship. That swung the raging debate on whether Obama or another senior American should go beyond the expression of regret that the administration had already given, and apologise," the report said.

Pakistani officials have said in the absence of an apology, they cannot reopen NATO supply routes into Afghanistan that have been closed since November.

In turn, Americans are withholding between USD 1.18 billion and USD 3 billion of military aid. The continuing deadlock also does not bode well for Pakistan’s attendance at a NATO meeting in Chicago next month assuming it is even invited.

Obama administration officials said the stalemate would not be resolved quickly. "This is the beginning of the re-engagement conversation," Victoria Nuland, the State Department spokeswoman, said in Washington.

"We're going to have to work through these issues, and it’s going to take some time." In an US election year, an American apology appears problematic given the Republican pressures on Obama and the hostility of a Congress with little patience for Pakistan.

"The politics of election year in both countries are slowing down the resolution of admittedly vexed issues in an environment of persistent mistrust," said Maleeha Lodhi, a former Pakistani ambassador to Washington.

According to new details about the attacks emerging in the past two weeks, some fighters were smuggled into Afghanistan over time and in small numbers, many may were brought in from Pakistan only a couple of days before the attacks, said a senior American military officer in Afghanistan.

"Our initial assessment is they probably moved them in a last moment to avoid detection," said the officer.

The reopening of NATO supply lines is important for the United States military to support troops currently in Afghanistan, as well as to help withdraw tons of weapons out troops begin to withdraw in 2014.