UN Secretary General Antonio Guterres on Sunday urged warring parties in Yemen to allow humanitarian aid into the country amid a political stalemate that has seen violence spill into Sanaa.

"We are doing are best to create the conditions for the present stalemate to be overcome," Guterres said after talks in Kuwait, which is leading mediation efforts in crises across the region.

His comments come after tension between Yemeni ex-president Ali Abdullah Saleh and his ally Abdul Malik al-Houthi, who control the capital Sanaa, escalated into armed clashes that left two rebels and a pro-Saleh colonel dead late Saturday.

Since 2014, the Saleh-Houthi alliance has fought the UN-recognised government of Abedrabbo Mansour Hadi for control of the impoverished country.

Witnesses in the capital said the ex-president's forces had spread in southern parts of the city near the presidential offices, which Saleh still holds despite resigning in 2012.

A Saudi-led coalition supporting the Hadi government imposed an air and sea blockade on all rebel-held territory in March 2015 and tightened it in August last year saying it was the only way to stop weapons smuggling.

Guterres said the UN was trying to facilitate the re-opening of the country's main international airport in Sanaa as well as the Hodeida port, a key entry point for aid also in rebel-held territory.

"We will be working very closely with the (parties) to see when and how a new strong initiative will be possible," he told a news conference.

More than 8,400 civilians have been killed and 47,800 wounded since the Arab coalition joined the war in 2015, according to the World Health Organization.

Another 2,000 people have died of cholera in a deadly outbreak that has spread across Yemen since April.

Long the poorest country in the Arab world, millions of Yemeni now stand at the brink of famine, according to the United Nations.