Several thousand Afghans protested in the eastern city of Jalalabad today over the killing of several civilians by US forces.
Afghan officials say US marines shot dead at least 10 people during a gunfight outside Jalalabad after a suicide attack at the weekend.
The US military will only say 16 people died in the suicide bombing and subsequent firing.
Human Rights Watch's Asia director, Brad Adams
"Suicide bombers in Afghanistan regularly pose as civilians, but that doesn't give coalition forces carte blanche to respond with indiscriminate fire," said Brad Adams of Human Rights Watch (HRW).
"The fact that the insurgents violate the laws of war doesn't absolve the US and its allies of the need to observe them."
He also said HRW was "concerned that the US military is attempting to control information" about the incident, saying the military confiscated the cameras of local and foreign journalists and deleted images and footage.
The Afghan government has opened an inquiry, but similar investigations by the government, Nato and US forces have done nothing more than confirm initial witness accounts.
Yesterday, a US bomb killed nine civilians - five women, three children and a man - near Kabul after a post was attacked.
Afghan leaders say foreign soldiers need to work more closely with local soldiers and police to be able to tell militants from ordinary people and minimise civilian deaths.