Protestors, who are demonstrating against a NATO cross-border attack, burn an effigy representing the US in Karachi November 27, 2011. [Photo/Agencies]

ISLAMABAD/KABUL  - Fury spread in Pakistan Sunday over a NATO cross-border air attack that killed 24 Pakistani soldiers and could undermine the US effort to wind up the war in Afghanistan.

Sunday night in Pakistan, more than 40 hours after the incident, many questions remained.

NATO described the killings as a "tragic unintended incident" and said an investigation was underway. A Western official and an Afghan security official who requested anonymity said NATO troops were responding to fire from across the border.

It's possible both explanations are correct: that a retaliatory attack by NATO troops took a tragic, mistaken turn in harsh terrain where identifying friend and foe can be difficult.

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"All of this is extremely murky and needs to be investigated," said a US official in Washington, who spoke on condition of anonymity. "Our goal today is ... that the investigation gets mounted in a way that is confidence-building on all sides."

Militants often attack from Pakistani soil or flee after combat across a porous border that NATO-led troops, under their United Nations mandate, cannot cross.

What is clear is the incident could undermine US efforts to improve ties with Pakistan so that the regional power helps stabilise Afghanistan before NATO combat troops go home by the end of 2014.

The attack was the latest perceived provocation by the United States, which infuriated Pakistan's powerful military with a unilateral special forces raid that killed al Qaeda leader Osama bin Laden in May.

Thousands gathered outside the American consulate in the city of Karachi to protest against the NATO attack.

A Reuters reporter at the scene said the angry crowd shouted "Down with America." One young man climbed on the wall surrounding the heavily fortified compound and attached a Pakistani flag to barbed wire.

"America is attacking our borders. The government should immediately break ties with it," said Naseema Baluch, a housewife attending the demonstration. "America wants to occupy our country but we will not let it do that."

Pakistan buried the troops killed in the attack Sunday. Television stations showed coffins draped in green and white Pakistani flags in a prayer ceremony at the headquarters of the regional command in Peshawar, attended by army chief General Ashfaq Kayani.