A moderate earthquake measuring 4.2 on the Richter scale rattled Turkey's biggest city Istanbul on Monday, causing a short panic but no initial reports of damage or injuries.

The Kandilli Observatory in Istanbul said that the tremor struck at a depth of 7.7 kilometres (4.8 miles) in the Sea of Marmara, to the south of the city, at 5:45 pm (1545 GMT).

The quake was felt throughout Istanbul, a metropolis of some 16 million that serves as the country's financial and cultural hub.

Istanbul residents felt the ground shift, causing a brief panic, an AFP correspondent said.

Turkish and foreign seismologists agree that Istanbul could be struck by major earthquakes in the coming decades due to its location less than 20 kilometres from the North Anatolia faultline.

Some 20,000 people were killed in two massive quakes that devastated Turkey's densely populated northwest -- including parts of Istanbul -- three months apart in 1999 as the eastern strand of the faultline ruptured.