Villagers shout anti-American and anti-Afghan government slogans during a Tuesday funeral for people killed in a pre-dawn NATO operation, in Mehtar Lam, Afghanistan, some 100 kilometres east of Kabul. (Rahmat Gul/Associated Press)

The No. 2 commanding general in Afghanistan says NATO is investigating the possibility of civilian deaths in a NATO-led attack in eastern Afghanistan on Tuesday.

Lt.-Gen. David Rodriguez told reporters Wednesday that the attack, which involved coalition and Afghan troops, was a "confusing operation" and "possibly" resulted in civilian deaths.

"We're continuing to investigate," he said.

The Afghan presidential palace said on Tuesday that six civilians, including one woman, were killed during an overnight strike outside Mehtar Lam, the capital of the eastern province of Laghman.

Provincial officials said civilians were among the 12 people killed in the clash, but did not specify numbers.

A NATO spokesperson had said Tuesday that no reports could substantiate the claims civilians were harmed, and that seven militants were killed and four were detained.

About 400 people took to the streets in Mehtar Lam to protest the deaths, carrying bodies of some of the dead, said provincial government spokesman Sayed Ahmad Safi.

The protest turned violent as the demonstrators clashed with police and one protester was killed, Safi said.

With files from The Associated Press