Most of the injured suffered broken bones as they jumped from buildings.
The 2:55 p.m. quake had a preliminary magnitude of 5.8 and was centered on Izmit, about 50 miles southeast of Istanbul, the city's Kandilli Observatory reported.
The U.S. Geological Survey in Golden, Colo., also reported that the quake registered a preliminary magnitude of 5.8, and called it an aftershock to the August quake.
Â"It was the strong aftershock which had been expected,Â" agreed Aykut Barka, professor of geology at Istanbul Technical University.
Istanbul Deputy Governor Ali Cafer Akyuz said three people were killed in the town of Kocaeli when a building collapsed on their car. Another three were killed in the same town when a building fell on them, he said. Additionally, one woman died of a heart attack in the town of Adapazari.
Prime Minister Bulent Ecevit said 239 people were injured, 15 buildings in Izmit collapsed and seven in Golcuk fell down.
Ecevit said rescue teams were rushing to the region to try to save people who may be trapped under the rubble, and urged people to be calm.
Â"These aftershocks may continue,Â" he said. Â"We hope that none are as powerful as this one. We unfortunately have to live with them.Â"
Injuries were reported in the towns of Adapazari, Kocaeli, Golcuk and Izmit, which had suffered serious damage in the August quake.
Today's jolt was felt as far away as Istanbul, where people rushed into the streets in panic. Telephones lines in Istanbul were down after the quake.
Last month's quake, which registered 7.4, killed more than 15,000 people and devastated western Turkey, the country's industrial heart.
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