Media playback is unsupported on your device Media caption The Vietnamese Coast Guard has released footage showing what they say are Chinese ships intentionally ramming Vietnamese vessels

The US has spoken out over "dangerous conduct and intimidation" in the South China Sea, after ships from Vietnam and China collided in disputed waters.

The collisions came as the Vietnamese ships tried to prevent China setting up an oil rig near the Paracel islands.

The incident is the most serious between the countries at sea in years, with dozens of boats now in the area.

The US state department called Beijing's move to introduce an oil rig in the area "provocative".

"This unilateral action appears to be part of a broader pattern of Chinese behaviour to advance its claims over disputed territory in a manner that undermines peace and stability in the region," spokeswoman Jen Psaki said in a statement.

"We are also very concerned about dangerous conduct and intimidation by vessels operating in this area," she said, calling on all parties to operate in a "safe and professional manner".

Paracel Islands Called Xisha in Chinese, Hoang Sa in Vietnamese

Claimed by China, Taiwan and Vietnam

More than 30 islands and reefs, including two main groups: the Amphitrite group and the Crescent group

Woody (Yongxing) Island, the largest island in the archipelago, now hosts the City of Sansha with a small community of fishermen, civil servants and soldiers

Vietnam maintains 'historical claims' to the Paracels without physical presence there

It controlled several islands within the Crescent group, where it had a weather station, until 1974, when after a brief but bloody clash China gained control over the entire archipelago

The events, she added, highlighted the need for claimants to disputed areas to clarify their claims in accordance with international law.

The incidents - of which there are reported to have been at least three - took place close to the Paracel islands, which are controlled by China but also claimed by Vietnam.

They came after China announced last week that it was moving a drilling rig into the area.

Vietnam said it had sent maritime police and fisheries vessels, and showed footage at a news conference of Chinese ships ramming its vessels. Six officials had been injured, it said.

China, meanwhile, said the "disruptive activities by the Vietnamese side are in violation of China's sovereign rights".

Meanwhile, Japan's Chief Cabinet Secretary Yoshihide Suga told reporters on Thursday that Japan - which is also involved in a territorial dispute with Beijing over East China Sea islands - was "strongly concerned about heightened tensions in the region due to China's illegal exploration".

"We urge [China] to refrain from taking unilateral actions that would escalate the tensions, and exercise restraint in accordance with international law," Mr Suga said, according to Japanese news agency Kyodo.

China claims a U-shaped swathe in the South China Sea that overlaps areas claimed by several of its neighbours.

The Philippines is currently taking China to the UN court over maritime territorial disputes between the two.

Image copyright AP Image caption There have been at least three incidents reported between Vietnam and China in the disputed waters

On Wednesday, Philippine police seized a Chinese fishing boat and detained its 11 crew in another disputed part of the South China Sea, prompting a protest from China.

China has called for the immediate release of the fishermen and their boat. During the daily press briefing on Thursday, Foreign Ministry spokeswoman Hua Chunying warned the Philippines to "make rational explanations" and "take no more provocative action".

She added that a Chinese maritime police boat had arrived in the area.

Meanwhile, the Philippines said the fishermen would be investigated to see if they broke the law, including one on endangered species. Several hundred turtles were reportedly found in their boat.

National Police Chief Alan Purisima told media the fishermen's arrest was made in Philippine territory.