The United Arab Emirates condemned the "occupation" of its embassy in Sanaa by Yemen's Iran-backed Huthi rebels, the foreign ministry said in a statement demanding their immediate exit from the compound.
The embassy takeover comes as Yemeni loyalists backed by the UAE and Saudi Arabia continue advances against rebels in several southern provinces as well as in third city Taez, seen as a gateway to the rebel-held capital.
"This act is further evidence that the group that committed this attack does not show any regard or respect for international conventions and diplomatic norms, as it practises the law of the jungle," said a statement published late Monday by the official WAM news agency.
It "condemned in the strongest possible terms" the rebel storming of the embassy, which it said took place on Sunday.
The foreign ministry "stressed that the occupation of the embassy and the eviction of its staff will not deter the UAE's support for the restoration of stability to sisterly Yemen."
The six-nation Gulf Cooperation Council, of which the UAE is a member, also released a statement condemning the "cowardly act."
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The Huthi rebels, allied with troops loyal to former president Ali Abdullah Saleh, seized the Yemeni capital in September last year.
They consolidated their grip on power in February, prompting an exodus of foreign diplomats from the capital.
The UAE shut its embassy in Sanaa and announced it would open another in southern city Aden, which was later the scene of intense fighting between local government supporters and the rebels before loyalists retook the city in mid-July.
It is unclear how many employees were at the Sanaa embassy and if it had been operational when it was stormed.
Saudi-owned newspaper Al-Hayat has reported that 1,500 troops, most of them from the UAE, had entered Aden in support of loyalists.
Official Emirati media says that six UAE soldiers have so far been killed in incidents linked to the Yemen fighting.
The UAE is also part of a Saudi-led coalition that launched an air war against the rebels on March 26.