Witnesses in Yemen say Wednesday's army air raid on a camp for displaced people is believed to have killed some 87 civilians.
News reports quoted camp residents accusing the military of gunning down civilians who were sleeping under trees and in makeshift tents in the camp in the northern area Adi.
The region is close to the front lines of intensifying fighting between government forces and Shiite rebels.
The Yemeni government has not commented on the reported attack. The army earlier reported inflicting heavy casualties on suspected Shi'ite rebels in the area.
New York based Human Rights Watch urged Yemen to investigate and stressed that civilians should never be targeted.
Yemen's government has been fighting Shi'ite rebels from the Zaidi sect intermittently over the past five years. The government launched its latest offensive last month, after the rebels said they seized control over more territory in the northwest.
The government says the Shi'ite rebels want to restore a Shi'ite state that fell in the 1960s. Rebels have rejected a truce offer
Last month, Yemen's president vowed to crush the Shi'ite rebellion, calling it a "cancer" that needs to be "uprooted."
He said the military would buy all of the weapons needed to strike the rebels wherever they are.
Some information for this report was provided by AP and Reuters.