A NATO Black Hawk helicopter came down in southern Afghanistan, killing seven American soldiers and four Afghans, the military said, as Taliban insurgents claimed responsibility.
The four Afghans included three members of the security forces and a civilian interpreter, NATO's US-led International Security Assistance Force (ISAF) said.
"The cause of the crash is under investigation," it said, adding that the helicopter was a UH-60 Black Hawk. The statement gave no further details.
Taliban spokesman Qari Yusuf Ahmadi said: "Our mujahideen (holy warriors) shot down an ISAF helicopter in Chenarto area of Shah Wali Kot district in Kandahar province at around 11:00 am (06:30 am GMT)."
He said a rocket-propelled grenade had been used against the helicopter.
"The helicopter was destroyed and all the crew and soldiers inside were killed," Ahmadi said.
"The NATO helicopter was hit by a Taliban rocket in Khashir area of Chenarto village in Shah Wali Kot district this morning," a local official who requested anonymity said.
The area had been cleared of Taliban in a push by NATO and Afghan forces in 2010, but the insurgents had become active in the district again, district governor Obaidullah said.
"Taliban have been active in Chenarto village since the beginning of this year. Afghan and foreign forces have had frequent clashes with the Taliban in this district since the beginning of this year," said Obaidullah, who uses just one name.
The ISAF statement did not use its normal phrasing for a simple helicopter crash, which includes the line that no enemy activity was reported in the area.
Helicopter crashes are fairly frequent in Afghanistan, where the 130,000-strong NATO mission relies heavily on air transport.
In a separate incident on Thursday, an Afghan army vehicle was hit by an improvised explosive device in the north of the country, killing at least seven Afghan soldiers, officials say.
The blast took place in a central district of Baghlan province early on Thursday morning, and targeted the vehicle while soldiers were on patrol.
The casualty toll was confirmed to Al Jazeera by General Janullah Safi, the local Afghan military commander.
Two other roadside bombs were reported in Baghlan last month, including one that killed four Afghan policemen. In the other attack, a convoy of vehicles transporting the Afghan minister of higher education hit a roadside bomb while travelling in the province. The minister was not riding in the
vehicle that hit the bomb, but two of his bodyguards were wounded in the blast.
Thursday's attacks came after 27 people were wounded in attacks on civilians in Herat and Khost provinces on Wednesday, and 48 hours after at least 55 Afghans were killed in a triple suicide bombing in Nimroz province on the Iran border.