They have been taken to the Salif port in Hodeidah, Yemen, the officials said.

The two Koreans were aboard a tugboat and a drilling rig that belong to the South Korean firm Woongjin Development Co. The rebels also seized another tug boat of Saudi Arabian nationality that is chartered by a company of the United Arab Emirates.

Reports say that the Houthi rebels claim that the vessels violated their territorial waters.

Mohammed Ali al-Houthi, a senior Houthi official, told Reuters that the Yemeni Coast Guard is checking to see whether the seized ship belongs to the countries of aggression or to South Korea, "in which case it will be released after completing legal procedures."

Seoul officials were first informed of the seizure after one of the two South Koreans sent a social media message to their company.

South Korea deployed its anti-piracy Cheonghae Unit, which has been operating off the coast of Somalia, to back the rescue efforts. The unit's destroyer is expected to arrive at the scene Thursday.

Seoul has also asked the United States, Saudi Arabia and other countries to cooperate in the rescue efforts. (Yonhap)