SANAA/ADEN: A 72-hour truce in Yemen came under pressure on Thursday when missiles fired from Yemen killed two civilians in Saudi Arabia, the United States said, while a Saudi-led coalition launched air strikes that Iran-allied Houthi fighters said killed three people.

Saudi Arabia and its Gulf Arab allies have been embroiled in a conflict in Yemen since March 2015, fighting on behalf of an exiled Yemeni government against the Houthi group, which controls the capital Sanaa.

A UN-brokered ceasefire took effect late on Wednesday, raising hopes of an end to a war that has devastated the Arab world's poorest country and left it on the verge of famine.

That brought Sanaa its first night without air strikes in nearly three months, and the truce was generally holding across the Arabian Peninsula state, residents and officials said.

However, rockets were fired by the Houthi group at Jazan and Najran in Saudi Arabia, the Saudi-led military coalition said in a statement. It said that "43 violations were committed along the border ... in which snipers and various weapons were used, including missiles."

In Washington, US Secretary of State John Kerry made a plea for the Houthis to respect the ceasefire after he met with Saudi Foreign Minister Adel al-Jubeir. Kerry said a rocket attack from Houthi territory killed two Saudi civilians, though it was not immediately clear where the deaths occurred.

"It is essential that the Houthis, who have said they will support this ceasefire, live by it," Kerry told reporters.

"So any such breaches of this put at risk the entire possibility of getting back to talks," Kerry added, saying the ceasefire needed to be given time to take hold.

Kerry said Saudi Arabia "has a right to be free from missiles being launched from Yemen" and he called on the Houthis to draw back their troops and missiles.

The Houthis said they had launched attacks on Saudi military camps across the border over the past two days and that a coalition air strike on Thursday killed three civilians in northern Saada province.

In the southern city of Zinjibar, al-Qaeda militants ambushed a checkpoint and killed five soldiers and injured several others, a local official told a sign of how the war has spiralled out of control.

Coalition aircraft had bombed Sanaa every night since Aug. 7, residents said, starting after peace talks with the Houthis and forces loyal to ex-president Ali Abdullah Saleh broke down.

Several previous ceasefires have failed to pave the way for an end to the conflict, although they have significantly slowed fighting in a war that has killed at least 10,000 people.