KABUL: Afghan and foreign troops will halt offensive operations in the war against Taliban-led insurgents for the UN's International Day of Peace on Monday, the defence ministry said.

But US President Barack Obama expressed "serious" concerns about Afghanistan's elections adding to international misgivings about the future of the strife-torn nation.

Three US soldiers were killed in two incidents reported Sunday by NATO's International Security Assistance Force (ISAF).

And one day after Taliban leader Mullah Omar warned foreign troops of their "imminent" and "unequivocal defeat", the defence ministry said troops will remain on the defensive for Peace Day.

"Afghan troops will not attack rebel positions unless attacked," spokesman Mohammad Zahir Azimi told AFP. "The people of Afghanistan are more thirsty for peace than any other nation."

The directive follows a similar order from US General Stanley McChrystal, which falls short of a ceasefire by the more than 100,000 US and NATO troops he commands.

Troops will be on the defensive but "will not conduct offensive military operations on September 21 to observe the United Nations' International Day of Peace", McChrystal said in a recent statement.

The UN day was created in 1981 and in 2002, September 21 was declared the annual date for "commemorating and strengthening the ideals of peace both within and among all nations and peoples".

UN spokesman Aleem Siddique said that with international and Afghan troops on board, and amid "mixed messages" from the Taliban, he was hopeful of a significant drop in violence on Monday.

"We are unsure if the Taliban have committed to observing Peace Day," he said, adding: "This day is not about politics, it is about humanity."

Peace Day coincides with the three-day Eid festival that began Sunday, bringing an end to the Ramadan month of fasting.

Qari Yusuf Ahmadi, purportedly a Taliban spokesman, was quoted on ReliefWeb website as saying: "Our forces will remain in defensive position, as usual."

Foreign forces in Afghanistan, experiencing their deadliest year since the war began eight years ago with more than 350 deaths so far in 2009, are sceptical the rebels will keep their word.

"The Taliban said they would not conduct any operations during Ramadan, but six Italian soldiers are dead and a lot of people have been killed in suicide bomb attacks across the country," an ISAF spokesman said.

A massive suicide car bomb attack on an ISAF convoy in Kabul on Thursday killed six Italians, whose bodies were repatriated to Rome Sunday, and at least 10 Afghans.

In the southern city of Kandahar -- the Taliban's base when they held sway from 1996-2001 -- a bomb killed four civilians as crowds descended on shopping districts to prepare for the Eid festival on Saturday evening.

President Hamid Karzai, beleaguered by accusations of fraud in the August 20 election, used his Eid message to extend an olive branch to militants trying to overthrow him.

"On this auspicious day once again I ask all those Afghan brothers who are unhappy or are in others' hands to stop fighting, destroying their own land and killing their own people," he said.

Karzai has said, should he win another five years in the country's top job, he will launch peace talks with the leaders of the insurgency that has intensified in recent months.

Preliminary results show him with around 55 percent of the vote, ahead of his nearest rival Abdullah Abdullah on 28.7 percent.

But US President Barack Obama warned on Sunday there were "serious issues" about how Afghanistan's election was conducted in some regions.

"It did not go as smoothly as I think we would have hoped, and there are some serious issues in terms of how the election was conducted in some parts of the country," Obama said on NBC's "Meet the Press".

The UN-backed arbiter the Electoral Complaints Commission has identified around 2,500 polling stations out of 24,183 that opened on August 20 where "clear and convincing evidence of fraud" merits a recount and audit.

But the Independent Election Commission says such an audit would take six weeks, and are urging an alternative to prevent a prolonged political crisis.

Karzai's comments followed an Eid statement by Taliban leader Mullah Omar, telling foreign troops to learn the lessons of history.

Omar, who is believed to be in Pakistan, said in his statement: "The more the enemy resorts to increasing forces, the more they will face an unequivocal defeat in Afghanistan."