JAKARTA (Reuters) - Indonesia and Vietnam will launch a joint investigation into a sea incident at the weekend, officials said, after reports that Vietnamese coast guards had tried to forcibly free five fishing boats and their crew detained in waters near the Natuna Islands.

An Indonesian sea patrol intercepted the Vietnamese fishing boats with 55 crew on Sunday after they crossed into Indonesian waters, said Rifky Effendi Hardijanto, secretary general of Indonesia’s Fisheries Ministry.

Media said a Vietnamese coast guard vessel had rammed and sunk one of the boats being escorted to an Indonesian base further south on the island of Batam.

“Whether it was rammed or not, we have agreed to investigate,” Hardijanto told a news conference, adding, “We have agreed to settle this incident through diplomatic means.”

Hardijanto denied reports that an Indonesian officer on board the sunken ship had been taken hostage by the Vietnamese coast guard in a bid to free its fishermen.

He said the officer had been rescued by the Vietnamese coast guard and could leave Vietnam as soon as the Indonesian government arranged transport.

Indonesian Fisheries Minister Susi Pudjiastuti met Vietnam’s ambassador in Jakarta on Tuesday to discuss the issue, Hardijanto added.

“The two countries’ diplomatic departments have contacted each other and worked together to handle it and make sure similar cases do not happen,” said Vietnam’s ambassador, Hoang Anh Tuan.

Indonesia had sought Vietnam’s assurance it would not enter its territorial waters again, Hardijanto said.

He said the countries would also discuss border issues, since Indonesia defined its border on the basis of the United Nations Convention on the Law of the Sea (UNCLOS), while Vietnam used the continental shelf.

Indonesia has been holding regular exercises and has stepped up its military presence in and around the Natuna Islands, close to disputed waters in the South China Sea.

There have been confrontations between Indonesian and Chinese vessels in waters near the Natuna Islands as regional tension rises over Beijing’s assertiveness in the busy waterway.

In March last year, a Chinese coastguard vessel rammed a Chinese fishing vessel to free it following its seizure by Indonesian authorities.