The United Nations Security Council on Sunday urged Yemen's Huthis to cede power, release President Abedrabbo Mansour Hadi and negotiate in "good faith," after the Shiite militia vowed to defy the body's "threats."
Yemen is a traditional US ally in the fight against Al-Qaeda, but the impoverished Arabian Peninsula country has descended into chaos since the Huthis overran the capital in September.
In another city they captured last year, Ibb in central Yemen, Huthis fired on hundreds of protesters to disperse them on Sunday, wounding several.
Militiamen also clashed with Sunni tribes east of the central city of Baida, which the Huthis have been trying to overrun as they extend their influence.
Tribal sources said at least 12 Huthis were killed, but there was no independent confirmation of the toll.
On February 6, the Huthis ousted the government and dissolved parliament, tightening their grip after Western-backed President Abedrabbo Mansour Hadi resigned in protest at their advance.
UN Secretary-General Ban Ki-moon has warned that Yemen is falling apart and called for Hadi's reinstatement.
Citing security concerns, nine Arab and Western countries shuttered their embassies in Yemen last week and evacuated diplomats.
In its resolution, the Security Council demanded that the Huthis "immediately and unconditionally" engage in "good faith" in UN-brokered negotiations, withdraw their forces from government institutions and relinquish power.
The text, adopted unanimously by all 15 council members, also demanded that the militia release Hadi, Prime Minister Khaled Bahah and other officials and activists under de facto house arrest or in detention.
All parties must "accelerate inclusive UN-brokered negotiations" and set a date for a constitutional referendum and elections, the resolution added.
It raised the possibility of sanctions, without going as far as Gulf countries, which have demanded coercive measures under Chapter Seven of the UN Charter.
- UN's first resolution -
Western diplomats said Russia, which is already under US and European sanctions over its annexation of Crimea and backing of rebels in eastern Ukraine, was reluctant to vote for sanctions.
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It was the council's first resolution on Yemen since the Huthis grabbed power in a move the United States and Gulf Arab countries described as a "coup."
The six-nation Gulf Cooperation Council said Saturday it would act if the rival factions in Yemen fail to resolve their differences, without elaborating.
The 57-member Organisation of Islamic Cooperation (OIC) voiced support for the GCC statement, and the Arab League said it will hold an emergency meeting on Yemen on Wednesday.
Huthi spokesman Mohammed Abdulsalam, quoted by the official, militia-controlled Saba news agency, insisted that "the Yemeni people won't cede power in the face of threats."
He denounced as "provocative blackmail" demands for the Huthis to relinquish power and criticised the withdrawal of ambassadors.
- Accusations of torture -
In their bid to establish authority across Yemen, the militiamen have tried to stifle opposition and have been accused of detaining and torturing opponents.
They announced a ban on anti-Huthi protests last week and have repeatedly used live ammunition to disperse demonstrations in Sanaa and Ibb.
The family of one protester detained by the Huthis last week said he had died late Friday of torture wounds suffered in captivity.
Another two demonstrators held with him are in hospital after being found wounded and left on a street.
On Sunday, several protesters were wounded in Ibb when the Huthis fired live rounds to disperse hundreds of people demanding the release of an activist, witnesses said.
Ahmed Hazzaa, a leader of the anti-Huthi Rafdh (rejection) Movement, was detained on Saturday by Shiite militiamen, members of his group told AFP.
The Huthis are accused of receiving support from Shiite-dominated Iran, which criticised the "hasty action" of closing embassies in Sanaa, and insisted the Huthis were fighting "corruption and terrorism."
Meanwhile, political forces and governors of three southern provinces -- Aden, Lahij, and Mahra -- formed a local leadership group rejecting the Huthi "coup."
In a statement issued at the end of a meeting in the south's main city Aden, leaders of the three provinces demanded Hadi's reinstatement and affirmed their support for Yemen becoming a federation based on the outcome of a national dialogue held last year.