Yemen's warring parties resumed face-to-face peace talks on Wednesday after a three-day break triggered by a walkout by the government delegation, the United Nations said.

It is only the second round of face-to-face talks in the hard-won negotiations to end a devastating conflict that has killed more than 6,400 people and displaced 2.8 million since March last year.

"The plenary session has started. All are present including the government delegation," the UN envoy's spokesman, Charbel Raji, told AFP.

The negotiations, which began on April 21, broke off on Sunday after the government delegation quit in protest at the apparent surrender of one of the few loyalist bases in the northern mountains to Iran-backed rebels.

UN envoy Ismail Ould Cheikh Ahmed said the two sides had agreed that a monitoring committee supervising an April 11 ceasefire would launch a fact-finding mission into the rebels' takeover of the Al-Amaliqa base in Amran province, one of their strongholds.

The committee will submit a report within 72 hours with practical recommendations that all sides pledge to carry out, Ould Cheikh Ahmed said.

Foreign Minister Abdulmalek al-Mikhlafi, who heads the government delegation, has demanded a rebel pullout.

The United Nations stressed the need to strengthen ceasefire monitoring committees on the ground, particularly in and around battleground third city Taez, where loyalist troops have been under siege for months, trapping tens of thousands of civilians.

Human Rights Watch, which has been deeply critical of alleged violations of the rules of war by the government and its supporters in a Saudi-led military coalition as well as the rebels and their allies, called for justice for the victims from the UN-brokered talks in Kuwait.

It urged the warring parties to "support international investigations, transitional justice and victim compensation as key elements of any agreement".

"The armed conflict in Yemen has been characterised by numerous violations of the laws of war by all sides, which have not been investigated nor have resulted in any redress for victims of unlawful attacks," the New York-based watchdog said.

Despite a Saudi-led military intervention in support of the government launched in March last year, the rebels and their allies still control the capital, as well as much of the northern and central mountains and Red Sea coast.

The United Nations says that most of the civilians killed in the conflict in the past 14 months have died in Saudi-led bombing raids.

HRW accused the coalition of carrying out "indiscriminate air strikes" against civilian areas even as rebels "committed various abuses."

"It's crucial for the Yemen peace talks to address past atrocities as well as future political arrangements," said HRW's Middle East director Joe Stork.

"A mechanism should be put in place to investigate abuses, prosecute those responsible and assist the victims," he said.