Six American soldiers serving with NATO forces were killed by a roadside bomb in Afghanistan on Sunday, the alliance's military in Kabul and an American official said.

"Six International Security Assistance Force service members died following an improvised explosive device attack in eastern Afghanistan today," ISAF said in a statement, without naming their nationalities or giving any further details.

In Washington, an official said the dead were Americans.

Another NATO soldier died in a similar attack earlier Sunday, ISAF announced, taking the day's toll to seven and the total toll for this year to at least 234, according to an AFP count based on the icasualties.org website.

As is traditional, ISAF deferred the identification of the casualties to their home countries.

NATO has some 130,000 troops in Afghanistan fighting Taliban who were ousted from power by a US-led invasion in 2001.

The Taliban are particularly active in the south and the east of the country, where NATO focuses most of its firepower ahead of its planned withdrawal from Afghanistan by the end of 2014.

Improvised bombs, often made with fertilizer, are a favorite weapon of the Taliban and account for a large number of overall NATO casualties.

They also take a high toll among civilians who use the same roads, with 18 being killed in three separate attacks in the southern province of Kandahar earlier Sunday.

For the past five years the number of civilians killed in the war has risen steadily, reaching a record 3,021 in 2011.

PHOTO CAPTION

US troops and Afghan National Army soldiers conduct a joint security dawn patrol in 2011.

AFP