A suicide bomber on a motorbike rammed an Afghan-NATO patrol in the town of Khost on Wednesday, killing 21 people, including three NATO troops, officials said.
Another 37 people were wounded in the blast in the eastern town close to the border with Pakistan, where Taliban and other Islamist insurgents fighting US-led troops have strongholds, hospital officials said.
It was the second major attack on NATO in Khost in three weeks. The government blamed "enemies of Afghanistan," a phrase commonly used by officials to refer to the Taliban.
The bombing will only heighten fears about security as NATO prepares to hand responsibility to Afghan forces and recall the vast majority of its 130,000 combat troops by the end of 2014.
The Taliban, leading a 10-year insurgency against the Western-backed government, has begun its annual fighting season with a series of attacks that forced US Defense Secretary Leon Panetta to admit that violence was rising.
Interior ministry spokesman Sediq Sediqqi said Wednesday's blast targeted a combined Afghan and coalition patrol passing through Khost, one of the most troubled parts of Afghanistan.
Khost shares a porous border with Pakistan's tribal belt, which lies outside government control, and where US officials say the Taliban and al Qaeda have carved out bases for operations in Afghanistan.
The Haqqani network, a militant group close to al Qaeda and blamed for some of the most daring insurgent attacks in Afghanistan, is particularly active in the province.
The US embassy in Kabul released a statement confirming that three members of the US-led NATO mission and an Afghan interpreter were killed.
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