MOSCOW — A fire on an oil platform in the Caspian Sea burned on Monday for a fourth day, and the Azerbaijani company that operates the site warned that the fire could spread to the oil wells that feed the platform, heightening the risk of a spill.

Workers were evacuated on Friday, but one of two lifeboats capsized in rough seas, leaving 29 people missing and presumed dead.

There have been no reports of spills so far. While rich in oil, the Caspian Sea, the largest enclosed inland body of water in the world, is also vulnerable to ecological damage.

After the breakup of the Soviet Union, Western companies including BP modernized the Caspian offshore industry. But the platform that caught fire on Friday was a legacy of the Soviet period, built in 1984.