Beijing  - The opening of Olympic security checkpoints on major roads leading into Beijing has caused long traffic jams and forced some drivers to sit in stationary vehicles for up to two hours, state media said on Wednesday.

Traffic came to a standstill on at least six main roads into Beijing on Tuesday, the first day of operation for the "three defence lines" of police checkpoints, the Beijing News reported.

A queue of vehicles stretched back about 2 kilometres from a checkpoint on the main highway linking Beijing and the nearby coastal city of Tianjin on Tuesday afternoon, the newspaper said.

One driver told the newspaper his car had remained stationary for two hours and that each security check appeared to be taking about 20 minutes.

The official Xinhua news agency also reported heavy traffic congestion on Tuesday along at least five other main roads into Beijing.

Following reports of the long tailbacks on Tuesday, the Beijing city public security bureau had requested the checkpoints to add staff and hand over suspicious people or vehicles to police as soon as possible to reduce delays to other travellers.

But the agency said a spokesman for the bureau had warned that delays and occasional traffic jams were "inevitable" during the security checks.

"We hope drivers and passengers could understand and support us in the coming two months," the unnamed spokesman was quoted as saying.

Police last week said they would use sniffer dogs, metal detectors, scanners and other electronic equipment at the checkpoints.

The "three defence lines" are an outer line of checkpoints on roads entering Beijing municipality, a second line of roads between the suburbs and the city proper, and a third line along the city's inner ring roads.

Police have the right to stop vehicles to search for "suspicious people and cargo," and could close entry points to the city if necessary, reports said.

All vehicles registered outside Beijing are subject to security checks and only allowed to enter the city with valid documents.

Tianjin and Hebei province, which surrounds both Tianjin and Beijing, were cooperating in forming the pre-Olympic security cordon, the government said.

The checks are part of a growing list of security measures introduced in recent weeks, including the deployment of anti-aircraft missiles near the Olympic Green.

The government plans to impose strict limits on the usage of Beijing-registered cars on the city's roads from July 20, and will encourage the city's 15 million people to use public transport during the Olympics.

China has also ordered tighter security checks at airports in its restive Xinjiang and Tibet regions, plus more than a dozen other cities. (dpa)