UNITED NATIONS, October 1 (RIA Novosti) - At least 1,100 people have lost their lives in two earthquakes in Indonesia, and many more are injured, the United Nations humanitarian chief said on Thursday.

The island of Sumatra was hit by a 7.6-magnitude quake on Wednesday, and by a second earthquake on Thursday with magnitude 6.9. Hundreds of buildings have collapsed, burying people under the rubble.

John Holmes told reporters at the United Nations headquarters: "The latest figures we have suggest the death toll has risen already to 1,100."

"These numbers, I fear, will rise as more information becomes available... It's still feared that thousands of people are trapped under damaged houses and many buildings," he said.

Earlier on Thursday, Health Minister Siti Fadillah Supari said the number of fatalities could reach several thousand.

Russia's emergencies ministry earlier said it would send rescue workers, doctors and a mobile hospital to Indonesia to help in rescue work.

The disasters are the latest in a spate of natural and man-made calamities to hit Indonesia.

Part of the Pacific's Ring of Fire, Indonesia is in the world's most seismically active region, registering 6,000-7,000 earthquakes with a magnitude of 4.0 or above annually.

On Wednesday, an earthquake thousands of kilometers to the west caused several tsunamis, which hit the Pacific island nations of American and Western Samoa, and Tonga, killing hundreds of people.