The UN Security Council has warned of unspecified further steps if the Shia Houthi rebels who took power in Yemen do not immediately return to talks.
Gulf Arab states had called for a stronger international response after the rebels announced a political takeover in the capital, Sanaa.
The rebels said they were dissolving parliament and appointing a council to act as president.
Rival Yemeni political groups denounced the move as a coup.
The Houthis took control of the capital Sanaa in September, forcing the resignation of President Abed Rabbo Mansour Hadi in January.
The country is also fighting an al-Qaeda insurgency with the help of US drones. Despite the takeover, the US said it was continuing to work with Yemeni on counter-terrorism.
Analysis: Sebastian Usher, BBC World Service
The declaration will be seen by many Yemenis as the final stage of a Houthi coup, although some may feel that it could offer some hope of greater stability. Government decisions will now in effect be dictated by a revolutionary committee, dominated by the rebels.
The Houthis delivered their message from the Republican palace in Sanaa to a huge gathering of political, military and tribal figures in an effort to show the range of their support.
Their writ will not be recognised by many Sunni and southern leaders, threatening Yemen with a further descent into chaos.
The UN's point man on Yemen, Jamal Benomar, headed straight to Saudi Arabia after the failure of talks to try to achieve some kind of political consensus there. It is a sign of how concerned the Saudis are over what they see as an Iranian-backed Shia takeover in a country where they're accustomed to calling the shots.
UN Security Council members in New York said they were "gravely concerned" and called on all parties to return to UN-led negotiations.
They also called for the immediate release of Yemen's president, prime minister and cabinet from house arrest.
The US state department said the Houthis' declaration did "not meet the standard of a consensus-based solution to Yemen's political crisis".
Ministers from the Gulf states of Bahrain, Oman, Qatar, the UAE, Saudi Arabia and Kuwait expressed concern about the developments.
"There was a feeling that the international community needed to take a stronger position," an official told Reuters on condition of anonymity. "There was concern about Iranian influence but no one discussed getting in touch with the Iranians."
Iran has been accused of giving financial and military support to the Houthis - something both have denied.
The Houthi announcement that they had taken power came after a deadline they set for political parties to resolve the crisis expired, and UN-brokered peace talks failed.
The new five-member council will run the country for a transitional period, the group said.
"[The aim is] to get the country out of the vacuum that was created by the sudden and unjustified resignation of the president of the republic and the government," said a Houthi rebel member, Jamal al-Swdi.
But there was condemnation from other Yemeni political factions: a member of the Sunni Islamist Islah party, Rashad Al-Sharabi, said the move "ensures that the situation in Yemen is heading towards a new conflict".
There have been reports of protests in several Yemeni cities, including Sanaa.
Abdel Aziz bin Habtur, the governor of Yemen's second largest city, Aden, called the announcement "a plot against the constitution", according to AFP news agency.
The situation in Yemen escalated last month when the Houthis seized a key aide of President Hadi, in an attempt to block a draft constitution.
They later took the presidential palace and other key buildings, prompting Mr Hadi's resignation. He said he could not continue in his post under such pressure.
He and and other ministers have been under house arrest since then.
Yemen has been riven by instability since protesters inspired by the Arab Spring forced the overthrow of President Ali Abdullah Saleh in 2011, who is believed to have been backing the Houthis.