RIYADH (AFP/REUTERS) - A ceasefire after weeks of Saudi-led coalition bombing in Yemen began on Tuesday night, but the coalition warned Iran-backed rebels that it would strike back at any violation.

"Absolutely. 11 O'clock", coalition spokesman Brigadier-General Ahmed al-Assiri said when asked whether the ceasefire had begun on schedule at 11pm local time (4am on Wednesday Singapore time).

"We are committed to respect this," he said, but the coalition will continue its "intelligence, reconnaissance and surveillance" in case it has to respond.

"We will be ready to react to any violation of the pause," he said.

"We are very clear. If they do not respect... we will continue."

The Houthis shelled Saudi border areas in Jizan province until the last moments before the ceasefire started, Asseri said on al-Arabiya television, adding that this gave him no confidence the rebel group intended to keep to the truce.

Any Iranian vessels sailing to Yemen needed permission from Yemeni authorities or the coalition to make the journey, he said.