BEIJING (AFP) - Beijing expressed "serious concerns" on Wednesday over protests in Vietnam that saw more than a dozen factories set on fire by anti-China demonstrators enraged over its deployment of an oil rig in disputed waters.

China had made "solemn representations" and asked Vietnam to take all necessary steps to stop and punish the crimes, foreign ministry spokesman Hua Chunying told reporters at a regular briefing.

"China expresses serious concerns about the incident," Ms Hua said. The foreign ministry "launched solemn representations with the Vietnamese ambassador to China, urging the Vietnamese side to immediately take effective steps to resolutely stop and punish these crimes, and to ensure the safety of Chinese citizens and institutions in Vietnam," she added.

China and Vietnam are locked in a long-standing territorial dispute in the South China Sea over islands and waters which both countries claim.

The tensions were ratcheted up last week when Beijing moved a state-owned, deep-water drilling rig for the first time into waters near the Paracel islands, a move Hanoi has condemned as "illegal".

There have been repeated skirmishes near the oil rig in recent days involving vessels from the two countries, with collisions and the use of water cannon and a rare outburst of public unrest in Vietnam, where authorities said 500 rioters have been arrested in connection with anti-China protests.