The United States warned Tuesday that talks on Yemen's two-month truce were in "trouble" as it pushed for an extension to help support millions at risk.

Yemen's Saudi-backed government and Iran-aligned Houthi rebels on April 2 entered a UN-brokered truce, the first since the brutal war broke out in 2016, but it expires on Thursday.

Talks to extend the pause in fighting "haven't ended yet but seem to be in a bit of trouble," said Linda Thomas-Greenfield, the US ambassador to the United Nations.

The talks impasse "is troublesome to us," she told reporters.

"We encourage the parties on both sides to continue those efforts and find a peaceful way to provide needed humanitarian assistance to the people of Yemen."

Secretary of State Antony Blinken also raised "efforts to strengthen and extend the truce in Yemen" during talks Monday with his Saudi counterpart, Foreign Minister Prince Faisal bin Farhan, the State Department said.

The war has killed hundreds of thousands of people and left millions on the brink of famine.

Aid groups have appealed for an extension of the truce, saying it has brought long-needed humanitarian dividends.

Earlier this month, the Norwegian Refugee Council said the number of civilian casualties in Yemen had dropped by more than half since the truce took effect.

The UN special envoy on Yemen, Hans Grundberg, last week also publicly urged progress and said that the truce had produced "tangible benefits."