Yemen's Iran-backed Huthi rebels have resumed an offensive to seize the government's last northern stronghold of Marib, a government source said Monday, with dozens of casualties on both sides. 

New clashes between the rebels and  pro-government forces -- backed by a Saudi-led military coalition -- erupted after weeks of relative calm in the oil-rich and strategic region.

The source told AFP that the Huthis had brought in reinforcements during that time. 

"Fighting took place about 10 kilometres (six miles) west of Marib," he said 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.

The Huthis have since 2014 held the capital Sanaa which lies just 120 kilometres (75 miles) 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.

The United States and Britain condemned the offensive, with the State Department saying it was "deeply troubled" by the violence and calling on the rebels to stand down and halt any new campaigns.

British Ambassador to Yemen Michael Aron said the Huthis must "prove their seriousness in wanting peace by supporting the efforts of UN special envoy Martin Griffiths".

US President Joe Biden last week announced an end to support for Saudi-led offensive operations in Yemen, as part of a major reset in foreign policy.

The state department has also 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.

UN envoy in Iran

Humanitarian groups were deeply opposed to the terrorist tag, saying it jeopardised their operations in a country where the majority of people rely on aid.

But Yemen's information minister, Moammar al-Eryani, said Sunday that "a reversal of the designation will complicate the Yemeni crisis and prolong the coup". 

In a sign of the renewed diplomatic activity after the US policy revamp, Griffith is visiting Iran for the first time this week to discuss the crisis.

Biden, who has also halted some weapons sales to Saudi Arabia, called Yemen’s war a "catastrophe" which "has to end".

But given the country's complex dynamics, a swift end to the conflict is unlikely, and  analysts say the challenge is finding a compromise acceptable to its myriad armed factions.

Without any wider political resolution that also takes in Iran, Saudi Arabia fears that walking out of Yemen will leave a dangerously unchecked insurgency free to build its strength right on the kingdom's southern border.

Since 2015 Saudi Arabia has been leading a military coalition to support Yemen's internationally recognised government, in a conflict that the United Nations says has resulted in the world's worst humanitarian crisis.