Dozens of people have been killed as fighting escalates in the oil-rich and strategic region east of capital Sanaa, government sources say.
Yemen's Houthi rebels have resumed an offensive to seize the government's last northern stronghold of Marib, a government source said, with dozens of casualties on both sides.
New clashes between pro-government forces — backed by a Saudi-led military coalition — and the rebels erupted on Monday after weeks of relative calm in the oil-rich and strategic region.
A government source told AFP news agency that the Houthis had brought in reinforcements during that time.
"Fighting took place about 10 kilometres west of Marib," the source told AFP on condition of anonymity.
"At least 20 government fighters were killed and 28 others injured," he said, adding that many rebels also died but no accurate count was available.
Marib fighting escalates
The Houthis have since 2014 held the capital Sanaa which lies just 120 kilometres away and are mounting a fierce campaign to take Marib.
In the past 24 hours, government forces repelled "five attacks," the source said, a day after three pro-government fighters were killed and four injured in a missile attack on their camp in Marib.
UN envoy in Iran for talks
Also on Monday, the United Nations special envoy for Yemen who arrived in Iran on Sunday for talks on the grinding war in the Arab world's poorest country met with Iranian Foreign Minister Mohammad Javad Zarif.
Martin Griffiths' "immediate priority" in Tehran is to push for a nationwide ceasefire, urgent humanitarian measures, and the resumption of the political process, his office has said.
Those goals repeatedly have proved elusive over years of ruinous war that have left the country deeply divided.
The talks are part of a broader effort to negotiate a political solution to the near six-year conflict pitting Iran-allied Houthi rebels against Yemeni government forces supported by a Saudi-led military coalition.
Delisting terrorist designation
The US State Department said on Friday it had formally notified Congress of its intention to revoke a terrorist designation against the rebels, which had been announced at the end of former president Donald Trump's administration.
The delisting move came a day after Trump's successor Joe Biden announced an end to US support for Saudi-led offensive operations in Yemen.
Humanitarian groups were deeply opposed to the terrorist designation, saying it jeopardised their operations in a country where the majority of people rely on aid, and that they have no choice but to deal with the Houthis, who control much of the north.
Saudi Arabia has been leading a military coalition since 2015 to support the internationally recognised government in a war that the United Nations says has resulted in the world's worst humanitarian crisis.