The White House on Thursday dismissed the notion of offering an apology to Pakistan over the NATO air strikes that killed 24 soldiers, insisting an inquiry was still ongoing as to how the men died.

"I, speaking for the White House and the president, offered condolences on behalf of him, the administration, the American people, for the tragic loss of life," White House spokesman Jay Carney told reporters. "It was a tragedy."

But he and other US officials have stopped short of offering any apology.

On Monday, Carney also said the weekend deaths, which provoked outrage in Pakistan and prompted Islamabad to review its alliance with the US, were a "tragedy," adding: "We take this matter very seriously."

"We mourn those brave Pakistani service members that lost their lives," the spokesman said.

The New York Times reported Thursday that US State Department officials had lobbied for President Barack Obama to make a formal statement of regret to the Pakistani people, to stem the damage to the key US-Pakistani relationship.

But according to the newspaper, which cited unnamed administration officials, the Pentagon balked at the idea, saying statements by other senior officials such as Secretary of State Hillary Clinton were sufficient.

Some administration aides also expressed concern that an apology by Obama would be used against him by his Republican adversaries in the run-up to next year's presidential election, the Times reported.

When asked why no formal White House apology was forthcoming, Carney said authorities were at the "early stages" of an investigation into the incident.

"So I think that the expression of condolences for the tragic loss of life conveys sincere sentiment... and it goes to the importance of the relationship that we have with Pakistan," the spokesman said.

Pakistan has insisted that the attack on its troops was unprovoked. The Wall Street Journal, citing Afghan and Western officials, has reported that NATO and Afghan forces called in the strikes after taking fire from the Pakistani side.