A suicide bomber at a base in Afghanistan's volatile east killed eight American civilians, US officials said, the worst loss of life for Americans in the country since October. Four Canadian soldiers and a journalist were killed by a roadside bomb in Afghanistan's west, Nato said.

The suicide attacker detonated explosives Wednesday at Forward Operating Base Chapman in Khost province near the Afghan border with Pakistan, killing eight American civilians and wounding others, US officials in Washington said.

"We mourn the loss of life in this attack, and are withholding further details pending notification of next of kin," US State Department spokesman Ian Kelly said.

An attacker wearing a suicide vest caused the explosion, according to a senior US official in Washington. Another senior US official in Washington said there were conflicting reports on the number of casualties, but that others were injured in the attack. The officials spoke on condition of anonymity because not all details about the incident had been confirmed.

A senior State Department official told The Associated Press on condition of anonymity that all of the victims were civilians. However, that could include military contractors and US intelligence officials. Nato said only that the base is used by provincial reconstruction teams, which consist of both soldiers and civilians, and other personnel.

In Kabul, a spokesman for the international coalition force in Kabul said no US or Nato troops were killed in the afternoon explosion. The attack was the bloodiest day for Americans since eight soldiers were killed in an insurgent attack on a base in eastern Afghanistan on Oct. 7.

In the south, Nato said that the four Canadian troops and a reporter embedded with their unit died when their armored vehicle hit a bomb while on an afternoon patrol south of Kandahar city, the biggest loss of life for Canadian forces in Afghanistan this year.

The Canadian Press identified the journalist as Michelle Lang, a 34-year-old health reporter with the Calgary Herald, who was on her first assignment to Afghanistan.

The military has not disclosed the names of the troops because relatives had not all been notified.

"We are all very saddened to hear this tragic news," Alberta Health and Wellness Minister Ron Liepert said in a statement. "Michelle covered health issues with professionalism, accuracy and thoroughness. She was tenacious in her quest to inform Albertans, and for her diligence she was very well respected."

Brigadier General Daniel Menard, commander of coalition forces in Kandahar, told Canadian Press early Thursday that the soldiers were conducting a community security patrol in order to gather information about daily life in the area and how to maintain security.

Wednesday was the second lethal strike against the Canadian force in a week. One Canadian soldier and an Afghan soldier were killed December 23 during a foot patrol in Panjwayi district of Kandahar province. According to figures compiled by The Associated Press, the latest casualties bring to 32 the number of Canadian forces killed in Afghanistan this year; in all, 138 have died in the war.

Separately on Wednesday, Nato questioned Afghan reports that international troops killed 10 civilians, including schoolchildren, in a weekend attack that prompted hundreds of angry Afghan protesters to burn an effigy of US President Barack Obama and chant "death" to America.

The head of an investigative team appointed by Afghan President Hamid Karzai told The Associated Press by telephone that eight students between the ages of 12 and 14 were among the dead discovered in a village house in a remote section of Kunar province in eastern Afghanistan. Nato said in a statement released late Wednesday night that while there was no direct evidence to substantiate the claims, the international force had requested and welcomed a joint investigation to reach an "impartial and accurate determination" of what happened in the attack.

Conflicting accounts of what occurred during fighting in Kunar's Narang district prompted an emotional outcry over civilian deaths, one of the most sensitive issues for international troops fighting the more than eight-year-old war. Although insurgents are responsible for the deaths of far more civilians, those blamed on coalition forces spark the most resentment and undermine the fight against militants. With 37,000 more US and Nato troops being deployed to the battle zone, concern over civilian casualties is unlikely to ease anytime soon.

Several hundred Afghans demonstrated in the capital of Kabul and in the eastern city of Jalalabad where the likeness of Obama, adorned with a small American flag, burned on a pole held above demonstrators.