A tourist bus caught fire Monday morning near Beijing, but the number of casualties were not immediately known, witnesses said.

It was also unclear how many people were traveling in the bus.

Photos posted by witnesses on the web showed the whole vehicle was engulfed in flames and dark smoke.

The accident happened at the juncture area between Beijing and Tianjin, on a pivotal expressway linking the capital with Shanghai.

Traffic police in Tianjin's Wuqing district were on their way to the site.