Saudi Arabia warned on Saturday it would back Yemen's government in any military confrontation with separatist forces and urged them to withdraw "peacefully" from recently-seized provinces.

The statements came a day after reported Saudi air strikes on separatist positions in Yemen's Hadramawt province – and after Washington called for restraint in the rapidly escalating conflict.

General Turki al-Malki, the spokesman for the Riyadh-led groups, said they would act "directly and at the appropriate moment... to protect civilian life", according to the Saudi news agency SPA.

Saudi Defence Minister Khalid bin Salman posted on X that troops from the separatist Southern Transitional Council (STC) should "peacefully hand over" two regional governorates to the government.

The STC had warned on Friday that they were undeterred after strikes blamed on Saudi Arabia hit their positions, in the latest escalation since they seized large swathes of territory last month. There were no immediate reports of casualties in the strikes.

In recent weeks, separatists backed by the United Arab Emirates and seeking to revive the formerly independent state of South Yemen have made territorial gains.

Experts say their successes have embarrassed regional heavyweight Saudi Arabia, the main backer of the internationally-recognised Yemeni government.

Farea al-Muslimi, from the Chatham House think tank in London, said the STC had crossed Riyadh's "red lines" and that the situation could quickly get worse.

"It is one bad thing to humiliate Saudi Arabia. And it is another much worse thing to humiliate Saudi Arabia publicly. And that's exactly what they did," he said.

A Yemeni military official said some 15,000 Saudi-backed Yemeni troops, massed at the edges of territory seized in recent weeks by the UAE-backed STC, had yet to receive orders to tackle the separatists.

Following Friday's raids, Yemen's government urged the Saudi-led coalition to support its forces in Hadramawt, after separatists seized most of the country's largest province.

Separatist advances have added pressure on ties between Saudi Arabia and the UAE, which support rival groups within Yemen's government.

The government is a patchwork of groups that includes the separatists and held together by shared opposition to the Iran-backed Houthis.

The Houthis pushed the government out of Yemen's capital Sanaa in 2014 and secured control over most of the north. (AFP)