A suicide car bombing in Kabul has killed at least 16 people, as Nato military chiefs were set to discuss a plea for more troops for Afghanistan.
The attack targeted a US military convoy
The attack on a US military convoy occurred near the American embassy, killing two US soldiers and injuring two others, the US military said.
It came hours before talks got under way in Poland, where Nato generals are expected to seek 2,000 more troops.
Nato forces in the south face mounting casualties in clashes with the Taleban.
MAJOR SUICIDE ATTACKS
28 August 2006: 17 die in bombing in southern province of Helmand
3 August 2006: 21 killed in bomb in Kandahar province
7 February 2006: 17 die in two attacks in Kandahar and Farah provinces
16 January 2006: Two attacks in Kandahar province kill 24 people
15 January 2006: Canadian diplomat and 2 Afghan bystanders killed in Kandahar
5 January 2006: 10 killed in Tirin Kowt, Uruzgan
September 2005: 12 killed outside Afghan army base in Kabul
June 2005: 20 killed in Kandahar mosque
The Kabul blast targeted a US military convoy hitting a Humvee, a military spokesman told the BBC.
Local hospitals told the BBC nearly 30 people had been wounded.
The BBC's Alastair Leithead at the scene said debris and body parts had been scattered over a wide area by the force of the blast, which was felt across the city.
Witness Najibullah Faizi, 25, told the Associated Press news agency that he saw a blue car ram one of two armoured vehicles in the convoy.
"I fell to the ground after the blast. American soldiers started shooting at another car nearby. There was smoke and flames everywhere," he said.
A man claiming to be a spokesman for the Taleban told an Afghan news agency that the rebel group was behind the attack.
Such claims have not always proved to be reliable, although the Taleban have carried out many attacks in Afghanistan.
There has been a series of suicide bombings across Afghanistan, but such a large explosion in the centre of Kabul is unusual.
Also on Friday, four Italian peacekeepers were injured, one seriously, in a roadside explosion near the city of Farah, the Italian defence ministry said.
And a suicide attacker struck a convoy near the southern city of Kandahar, although only the bomber died.
The violence comes as Nato commanders seek to boost numbers of troops, attack helicopters and transport aircraft as forces carry out Operation Medusa, a major offensive against Taleban militants in southern Afghanistan.
"We need to discuss the full range of our activity in Afghanistan," Gen Ray Henault, who is chairing the Nato talks in Warsaw, told Reuters news agency ahead of the meeting.
"This will include our troop level and the progress we have made there so far."
The talks are expected to continue until Sunday.
The BBC's Alix Kroeger at the Nato headquarters in Brussels says finding extra troops will not be easy - most Nato countries are already stretched by commitments in the Balkans, Africa and the Middle East.
Operation Medusa is the biggest in the area since the Nato-led International Security Assistance Force (Isaf) took over the area from a US-led coalition at the end of July.
Several Nato soldiers, most of them British or Canadian, have been killed in fierce fighting with Taleban guerrillas since the alliance extended its peacekeeping mission in the south a month ago.
News of the latest violence came as the commander of British forces in Afghanistan said his troops faced an "intensity and ferocity" of fighting that was "far greater" than that faced by British troops serving in Iraq.
Gen Ed Butler said his troops were being attacked up to a dozen times a day and at times faced hand-to-hand combat.
Nato's top commander, Gen James Jones, on Thursday said the alliance had been taken aback by the scale of violence in the region.
Nato troops took over leadership of military operations in the region from the US in July.
Many analysts believe there were serious shortcomings in the intelligence assessments that established the initial mission, and that harder fighting was to be expected.
The Taleban ruled Afghanistan until late 2001 when they were toppled by US-led forces in the wake of the 11 September attacks on New York and Washington.