The cabin unit of the seastead floats next to HTMS Rin, one of three naval vessels used to dismantle and remove the platform after it was removed from its base. The structure was expected to arrive in Phuket's deep-sea port Monday night. (Photo by Achadtaya Chuenniran)

PHUKET: The navy on Monday began dismantling a so-called seastead, the key piece of evidence in a security case being pursued against an American man and his Thai wife, and is expected to complete the task within the day.

A seastead is a permanent settlement on a structure located in an area of sea outside the jurisdiction of any country.

Phuket provincial authorities, meanwhile, said they expect to seek a court warrant for the arrest of Chad Elwartowski, 46, a bitcoin investor, and Supranee "Nadia" Thepdet, next week.

Both are accused of undermining the sovereignty of Thailand under Section 119 of the Criminal Code and the Immigration Bureau has started the process of revoking Mr Elwartowski's visa.

The couple allegedly appeared on social media claiming they were independent of the judiciary or law of any state including Thailand and urged others who were interested in seasteading to join them.

About 300 men and three naval ships were deployed in the operation supervised by Rear Adm Vinthanarat Kochaseni, deputy commander of the 3rd Naval Area Command.

The operation was originally scheduled to take place on Saturday but was postponed as authorities waited for advice from the platform developer on how to dismantle it without causing damage.

Under the revised plan, the octagonal cabin unit would be removed from its floating base-pillar and loaded onto HTMS Mannai instead of being towed to the shore.

The floating base would be towed by HTMS Rin and the two pieces would be kept at Phuket deep-sea port.

Rear Adm Vinthanarat said on Monday the operation would have to be carried out swiftly.

"As we already know, the structure is unstable and is not marked on navigation charts so it may pose a threat to ships in the area," he said.

The six-metre wide platform, developed and installed by Ocean Builders, lies about 14 nautical miles (26km) off the coast of Phuket.

Thai authorities insist it is in the country's contiguous zone over which the country has jurisdiction and rights.

Supoj Rodruang Na Nong Khai, deputy governor of Phuket, said on Monday forensic officials and investigators had also been deployed during the operation to collect evidence for the criminal investigation.