A suicide bomber on a motorbike has attacked a convoy of NATO troops in the eastern Afghan city of Khost, killing 17 people, hospital officials say, in this month's second attack on foreign forces there.
At least another 36 people were wounded in Wednesday's blast, including women and children, officials at the main hospital in the city, near the Pakistan border, said.
Police and witnesses at the scene reported US casualties.
Pentagon officials, speaking on condition of anonymity, said as many as three US troops were killed in the suicide bombing in eastern Afghanistan. The officials also said that the Afghans killed were a mix of civilians and security forces.
US media reported that more than 100 American soldiers were treated for injuries after that blast.
Khost is a main area of operations for the Taliban-linked Haqqani armed group which the US says has been behind a string of bombings in Kabul as well as attacks on foreign forces in the countryside.
Leon Panetta,the US defence secretary, blamed the group for the attack on the base and pledged to fight them.
The Taliban, leading a 10-year campaign against President Hamid Karzai's government, have begun the annual fighting season with a series of attacks which saw Leon Panetta, US defence secretary, admit that violence was again rising.
While overall civilian casualty rates have been dropping, violence continues to take its toll on both civilians and NATO forces.
According to the UN, the number of Afghan civilians has dropped 36 per cent so far this year, compared to last.
This is the first time the death toll has declined over several months since the UN started tracking the figures.
The senior UN envoy for the country, Jan Kubis, called the trend promising but cautioned that too many civilians were still being caught up in the violence as fighters battle both Afghan and foreign forces.
Kubis’ office said that 579 civilians were killed in the first four months of this year, down from 898 killed in the same period of 2011. The number of wounded dropped from 1,373 to 1,216 in the January to April period.
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Nevertheless, an overall increase in civilian casualties was reported this March by the United Nations Assistance Mission to Afghanistan (UNAMA).
In their Annual Report on the Protection of Civilians in Armed Conflict, the group reports that a decade after it began, the armed conflict in Afghanistan again incurred a greater human cost in 2011 than in previous years.
The UNAMA documented 3,021 civilian deaths in 2011, an increase of eight per cent over 2010 [2,790 civilian deaths] and a 25 per cent increase from 2009 [2,412 civilian deaths].
Anti-Government Elements caused 2,332 conflict-related deaths of Afghan civilians in 2011, up 14 per cent from 2010. 77 percent of all conflict-related civilian deaths in 2011 were attributed to Anti-Government Elements.
The Taliban and their allies are responsible for most civilian casualties, according to UN figures.
In the first four months of 2012, anti-government forces caused 79 per cent of civilian casualties and Afghan and foreign forces nine per cent, Kubis said in remarks in Kabul.
It was not clear who was responsible for the remaining 12 per cent of the casualties, he said.
The US-led NATO force is also responsible for hundreds of civilian casualties every year, mostly in air strikes aimed at fighters in Afghan villages.
After peaking at 711 in 2010, NATO casualties have been on a downward trend. According to icasualites.org, which tabulates coalition casualty counts using Pentagon figures, 2011 saw 566 NATO casualties, and thus far the total for 2012 is 205.
The months of Apirl and May each saw lower coalition casualty counts than the same months last year.
For June, 24 NATO forces have thus far been killed, which is a trend that will see a sharp decrease from last year's same month, which saw 66 NATO soldiers killed.
The current downward trend is likely due to NATO ground forces becoming less active as withdrawal plans slowly begin, as well as France beginning to withdraw its forces earlier than planned.
The Taliban have waged a bloody fight against Karzai's administration since they were removed from power in a US-led invasion in 2001.
NATO is due to hand over security duties to Afghans and the vast majority of its 130,000 personnel are to leave the country by the end of 2014, but there are fears over local forces' ability to maintain peace.