Yemen's government and Iran-backed rebels agreed on Tuesday to free half of the prisoners and detainees held by both sides, in the first breakthrough in peace talks that began last month.

The deal came during a meeting of the joint working group on prisoners and detainees formed by UN special envoy Ismail Ould Cheikh Ahmed.

There has been mounting international pressure to end the Yemen conflict, which the UN estimates has killed more than 6,400 people and displaced 2.8mn since March last year.

"It was agreed during the meeting to release 50% of the prisoners and detainees within the next 20 days," Mane al-Matari, media adviser to Yemen's foreign minister who heads the government delegation, told AFP.

A source close to the Houthi rebel delegation confirmed the agreement to release half of those held by both sides, describing it as "an exchange of prisoners".

The two sides will meet again on Wednesday to finalise the mechanism on how and when the exchange will take place, Matari said.

"The Yemeni government is committed to release all the prisoners as per the agreement," he said.

Matari estimated that their number is in the "thousands", but the rebel source said there may be only hundreds of prisoners involved.

Following a two-day interruption, the two delegations resumed face-to-face talks on Monday after mediation efforts and an appeal by the UN envoy.

Two other working groups, spanning military, security and political issues, also met on Tuesday but failed to make any progress on the thorny issues facing them, sources close to the two delegations said.

Those issues include implementing a UN Security Council resolution which orders the Houthis to pull out of territory they occupied in a 2014 offensive and surrender heavy arms they captured.

Conflict at 'crossroads'  

The two delegations remained far apart as the rebels demanded the formation of a consensus transitional government to handle other issues and the delegation of President Abedrabbo Mansour Hadi insisted he is the legitimate head of state, the sources said.

The Yemen conflict pits the Houthis and their allies loyal to former president Ali Abdullah Saleh against forces fighting to defend Hadi's internationally recognised government, supported by a Saudi-led Arab coalition.

Pro-government forces pushed the rebels out of five southern provinces last year with air and ground support from the coalition.

But the Houthis continue to control the capital Sanaa as well as large parts of the country's north and west, and the Saudi-led coalition has drawn strong criticism over heavy civilian casualties.

Ould Cheikh Ahmed warned on Monday that the conflict was at a critical juncture.

"There is no doubt that we are at a true crossroads. We are either moving towards peace or going back to square one," he said.

Hours after the talks resumed on Monday, Saudi air defences intercepted a ballistic missile fired from Yemen, the coalition said.

Before that, coalition aircraft hit a military base captured by rebels north of Sanaa, killing at least 11, a military official said.

Coalition spokesman Ahmed Assiri told Egyptian Dream television late Monday that if Kuwait peace talks fail "we will storm the capital Sanaa and decide the battle in Yemen," by military means.

The talks follow two failed peace attempts in June and December last year in Switzerland.

The Houthis exchanged prisoners with Saudi Arabia in March after unprecedented talks mediated by tribes along the frontier, where dozens have been killed in cross-border shelling.