Unidentified forces attacked a U.N. aid vessel off the main Yemeni port of Hodeidah at the weekend and started a fire in the engine room, port authorities said on Monday.
The United Nations aid chief, Mark Lowcock, confirmed there had been an incident but said it was now over and everyone was safe, without elaborating.
The vessel used by the U.N. World Food Programme (WFP) was attacked after delivering a shipment at Hodeidah, Yemen's Red Sea Ports Corporation said. The port is held by the Iranian-aligned armed Houthi movement which has taken large parts of the country in a three-year-old war.
A WFP spokeswoman said an unidentified armed group "abroad a skiff had opened fire and attempted to take over" the vessel that was some 60 km (38 miles)) off the coast of Hodeidah.
"Both the crew and the vessel are safe, with no injuries or obvious damage to the vessel," spokeswoman Bettina Luescher said in a statement sent to Reuters.
The ship was waiting in anchorage for permission to leave from a Saudi-led military coalition, the Ports Corporation added. The coalition is fighting the Houthis and controls the nearby Zubair island.
"The vessel traffic office received a distress call from the VOS THEIA at 1730 (1430 GMT) on Sunday, June 3, 2018 about a fire in the vessel resulting from an external attack," the Port Corporation said in a statement.
"There was an incident," Lowcock told Reuters in Geneva. "We don't know who's responsible. We're investigating and the incident is over."
Saudi Arabia, the United Arab Emirates and Sunni Muslim allies entered the war three years ago to try to drive back the Houthis, a Shia group that forced a Saudi-backed government into exile in 2014.
Last month the U.S.-backed coalition said it had foiled attacks by explosives-laden speedboats deployed by the Houthis against commercial vessels, including an oil tanker, in the Red Sea, and in April a Saudi oil tanker was hit off of Hodeidah, suffering limited damage.
A coalition spokesperson did not immediately respond to a request for comment on the boarding.
Coalition-backed troops have been advancing on Hodeidah, but it is not clear if they plan to seize the port, which handles the bulk of Yemen's commercial imports and aid supplies.
Asked about a possible coalition ground assault on the port, a lifeline for eight million Yemenis being fed by the world body, Lowcock said: "There's no port more important than Hodeidah. So anything which called into question the operation of Hodeidah would be a matter of deepest concern."
Riyadh says the Houthis are using the port to smuggle Iranian-made weapons, accusations denied by the group and Tehran. U.N. inspectors check ships to ensure they do not carry weapons.
Yemen's war has killed more than 10,000 people, displaced three million and pushed the impoverished country to the verge of starvation.