China says it has released 21 Vietnamese fisherman who were detained more than a month ago in a disputed area of the South China Sea.

China's official Xinhua news agency reported Friday the men were released after signing a written guarantee. Xinhua said the guarantee was a pledge “not to infringe on China's maritime rights, especially fishing in its territorial waters.”

The fishermen were detained in early March near the Paracel lslands, which are controlled by China, but claimed by Vietnam.

The release of the fishermen comes as Beijing is locked in a stand-off with the Philippines in another disputed area of the South China Sea.

China has become increasingly assertive about is maritime claims in the potentially energy-rich South China Sea, regularly intercepting foreign fishing boats and oil exploration vessels.

Beijing insists the entire 3.5 million-square-kilometer region is part of its territory. The Philippines, Malaysia, Brunei and Taiwan also claim parts of the sea.