Eighteen people have been killed and 29 injured after a bus with Romanian tourists fell into a deep river gorge in central Montenegro.

Police said the bus swerved off the Zdrijelo Bridge and plunged 40m into a ravine formed by the Moraca River below, 50km north of the capital Podgorica.

Romanian President Traian Basescu and Prime Minister Victor Ponta sent their condolences to the victims of what an official said was a "huge tragedy" for the country.

The crash occurred on a dangerous stretch of road during a rainstorm but the cause is not yet known. 

The injured include a child who was on the bridge when the crash happened. Four of those hurt are in a critical condition.

A military plane and two planes equipped with medical equipment are to take the casualties home to Romania.

The country's foreign minister is also due in Montenegro to assist in the aftermath of the tragedy.

Emergency workers faced difficult conditions to recover the victims and those injured from the deep ravine.

The winding road that leads from Serbia in the north through the Moraca canyon and then to the sea coast is notorious for traffic accidents because it is narrow and slippery in wet weather.

Although the small Balkan country has invested millions in infrastructure over the past decade, traffic accidents are frequent, especially in summer when tourists flock to its Adriatic coast.