Pakistani officials called the earthquake "a national tragedy."
The tremors were felt as far as the capitals of Pakistan, India and Afghanistan and continued for hours afterward. Communications throughout the region were down, the Associated Press reported.
Pakistan's chief army spokesman Maj. Gen. Shaukat Sultan said at least 18,000 had died and 45,000 were injured, a vast majority on the Pakistani side of Kashmir. Landslides and damaged roads and bridges are hampering rescue efforts.
"The damage and casualties could be massive and it is a national tragedy," Sultan told the AP on Saturday. "This is the worst earthquake in recent times."
The U.S. Geological Survey said the 7.6 was centered about 60 miles northeast of Islamabad in the forested mountains of Kashmir, the disputed Himalayan territory divided between India and Pakistan.
Sultan said the worst-hit areas were in Pakistani-controlled Kashmir, including Muzaffarabad, the regional capital, and the towns of Bagh and Rawalakot. The districts of Batagram, Balakot, Mansehra, Abbottabad and Patan in northwestern Pakistan also suffered serious damage, he said.
In the capital Islamabad, Pakistani President Gen. Pervez Musharraf said that the military would provide "all-out help," sending C-130 transport planes and helicopters to devastated areas.
The quake brought down a 10-story apartment building in Islamabad trapping people in the rubble. Some residents were Westerners, the AP reported.
Officials in Indian-controlled Kashmir said at least 240 people were killed, including 30 soldiers patrolling the area who were caught in a landslide.
Vijay Bakaya, Jammu-Kashmir state's chief secretary, said teams of doctors and Red Cross volunteers were traveling by road and on foot to remote mountainous areas.
The quake also reached parts of Bangladesh and the American military base in Bagram, Afghanistan, but no casualties or damage were reported.