"The Ethiopian army's answer to the rebels has been to viciously attack civilians in the Ogaden," said Georgette Gagnon, Africa director of the U.S.-based group.

"These widespread and systematic atrocities amount to crimes against humanity. Yet Ethiopia's major donors, Washington, London and Brussels, seem to be maintaining a conspiracy of silence around the crimes."

The group issued two reports that it said documented the attacks -- one based on witness accounts and the other using satellite imagery to show villages that had been burned.

Ethiopian government officials in Addis Ababa routinely reject allegations against their counter-insurgency operations in the rocky, arid region, which borders Somalia.

They also accuse the rebels of abusing locals but had no immediate comment on the new report.

Ethiopia, a key regional ally of the United States, launched its latest offensive after the Ogaden National Liberation Front (ONLF) attacked a Chinese-run oil field in the region in April 2007, killing more than 70 people.

In Washington, a State Department spokesman said the United States was aware of allegations of human rights abuses by Ethiopia. "However, we strongly reject Human Rights Watch's contention that the U.S. government has minimized or possibly actively ignored the matter," spokesman Gonzalo Gallegos said.

Aid organizations have reported that both the Ethiopian military and the ONLF were responsible for abuses, Gallegos said, adding that "it is not possible at this time" for U.S. or international observers to verify who had committed the abuses.

Human Rights Watch said one 130-page report was based on interviews by its researchers with more than 100 victims and eyewitnesses of abuses by soldiers.

The second was based on satellite images obtained commercially by the American Association for the Advancement of Science's Science and Human Rights Program.

The satellite images show eight sites, mostly in villages and small towns in the Wardheer, Dhagabur and Qorrahey Zones, that appeared to have been burned or destroyed recently.

For example, in the town of Labigah, 40 structures identified in a September 2005 image were gone in images taken in February 2008. In the Human Rights Watch report an eyewitness said the Ethiopian army "went into every village and set it on fire."

In the eyewitness report, Ridwan Sahid told how an Ethiopian soldier pushed him into a ditch and tried to kill him by taking a metal rod used to clean his gun and ramming it down his throat.

When Ridwan fought him off by twisting his fingers, more troops rushed over and tried to strangle him with a rope. Ridwan passed out and woke up later under the cold body of a friend.

Witnesses said at least 150 civilians were executed.

HRW said the government was limiting all access to the region, that the violence was ongoing and that staff believed their findings represented only a fraction of the actual abuses.

Gagnon said the army's tactics were fueling a looming humanitarian crisis and threatening the survival of thousands of ethnic Somali nomads who cross the area with their livestock.

Western nations give Ethiopia more than $2 billion a year in aid, she said, but must speak out now to halt the bloodshed.

"The government's attacks on civilians, its trade blockade, and restrictions on aid amount to the illegal collective punishment of tens of thousands of people," Gagnon said.

"Unless humanitarian agencies get immediate access to independently assess the needs and monitor food distribution, more lives will be lost."