The Iran-backed Houthis seized the ship on January 3, along with its 11-member crew, and then released a video which they said shows military equipment on board.
Yemen's Houthi rebels have rejected a UN request to release an Emirati-flagged vessel they seized earlier this month, saying it carried weapons.
"The Rwabee vessel was not carrying... toys for children but weapons for extremists," Houthi official Hussein Al Azzi said on Saturday.
The Iran-backed Houthis seized the ship on January 3 off the Red Sea port of Hudaida, along with its 11-member crew, and then released a video which they said shows military equipment on board.
The United Arab Emirates has described the Rwabee as a "civilian cargo vessel".
It said the ship was leased by a Saudi company and that it had been in international waters carrying equipment to be used at a field hospital.
'Act of piracy'
On Friday, the UN Security Council demanded the "immediate release" of the Rwabee and its crew.
It stressed "the importance of freedom of navigation in the Gulf of Aden and Red Sea", a strategic route for international shipping.
It also called on "all parties to de-escalate the situation in Yemen," including by working with the UN's special envoy to return to the negotiating table.
But Azzi, quoted by the Houthis' Al Masirah television, accused the UN of siding with "murderers who violate international laws".
The Rwabee "belongs to a country participating in the aggression against our people and at war with Yemen, and entered (Yemeni) territorial waters unlawfully", he said.
The UAE is part of the Saudi-led coalition which has described the seizing of the Rwabee as an act of "piracy".
Yemen's civil war began in 2014 when the Houthis seized Sanaa.
A Saudi-led military coalition intervened in Yemen to support the internationally recognised government in March 2015.
The UN has estimated the war killed 377,000 people by the end of 2021, both directly and indirectly through hunger and disease.