LUCKNOW ? (UPDATE) At least four people were killed when four express trains collided in separate incidents in the fog-bound northern Indian state of Uttar Pradesh on Saturday, police and railway officials said.

In the first incident, an inter-city express rammed into the back of a stationary passenger train near the town of Sarai Bhupat, 250 kilometers (155 miles) southwest of the state capital Lucknow.

Police director general Brij Lal said at least 10 passengers were injured.

Four people died in the second accident, 45 minutes later when two other expresses collided in similar fashion, some 80 kilometers (50 miles) southwest of Lucknow, Lal said.

"At least 35 others were injured in the impact, which damaged four carriages," Lal told AFP.

Senior railways official M.N. Srivastava said the impact in both cases had been minimized by the fact that the trains were travelling at low speeds because of the dense fog.

Visibility at both accident sites was down to around 30 meters (100 feet).

An estimated 2,000 passengers, mostly returning from year-end vacations, were travelling in the four trains.