The Saudi-led coalition fighting in Yemen said Iran-aligned Houthi rebels hijacked a vessel south of the Red Sea.
Saudi Arabia's state-run Saudi Press Agency quoted coalition spokesman Colonel Turki al-Malki as saying on Monday that Houthis seized the ship while it was towing a South Korean oil drilling rig on Sunday.
Al-Malki said the "attack" threatens vital shipping routes in the Bab al-Mandeb Strait, used for oil shipments from the Gulf to Europe, as well as goods from Asia to Europe.
"On Sunday, during the sailing of the tugboat Rabigh-3 in the southern Red Sea, it was hijacked and subjected to armed robbery by terrorist elements affiliated to the Houthi militia," said al-Maliki said.
He did not say which country the tugboat belonged to or how many crew members were on board the seized ship.
According the tracking website Marine Traffic, Rabigh-3's home port is in the western Saudi city of Jeddah and it sails under the kingdom's flag.
'Countries of aggression'?
Mohammed Ali al-Houthi, a senior rebel official, said: "Yemeni coast guards ... are checking to see whether [the vessel] belongs to the countries of aggression or to South Korea, in which case it will be released after completing legal procedures. The crews are being well treated. We assure everyone not to worry about the crew."
Al Jazeera's Mohammed al-Attab, reporting from the capital Sanaa, quoted a Houthi source as saying South Korea has demanded the release of its rig and crew.
"Houthis said they would release it after carrying out paperwork to confirm that this oil rig belongs to South Korea. The source told me the seizure is a usual measure that they are carrying out when they find any foreign vessel in the Yemeni regional waters," he said.
The Houthis have in the past targeted oil tankers and military ships belonging to Saudi Arabia and its partners in the military coalition fighting in Yemen's war since 2015.
In a meeting with the South Korean ambassador to Yemen, Yemeni Foreign Minister Mohammed Abdullah al-Hadrami sharply condemned the ship's seizure and called for its immediate release, according to a government statement.
"These provocative and illegal actions will negatively affect the freedom of international navigation," he said.
'A serious issue'
Gerry Northwood - from the international maritime security company MAST - said the rebels' ship seizure shows their strength at "insurgency-type warfare", particularly their ability to operate in the maritime domain.
"The Houthis have demonstrated over the last few years during the war that they do have the capabilities to do things and to disrupt, whether that be missile attacks, laying mines, or in this case taking a vessel.
"The fact that the Saudis can't necessarily protect vessels all the time in that area is quite a serious issue for them," Northwood told Al Jazeera.
The Saudi-led alliance intervened in Yemen in March 2015 against the Houthis after the group ousted the internationally recognised government from power in Sanaa.
Houthi forces have been driven away from most of Yemen's coast over the course of the conflict but still hold Hodeidah, the country's biggest Red Sea port and base of the group's navy.
The rebels have in the past targeted vessels off Yemen, which lies on one side of the Bab al-Mandeb Strait at the southern mouth of the Red Sea, one of the world's most heavily travelled oil tanker routes.
Al Jazeera and news agencies