Saudi-led coalition warplanes bombed Yemeni rebel targets including in the capital on Thursday following insurgent drone strikes on a key oil pipeline that Riyadh said were ordered by its arch-rival Tehran.
The new bombardment came after the UN envoy, who has been spearheading efforts to end more than four years of conflict in the Arab world's poorest country, warned it still faced the threat of plunging into all-out war.
The Saudi deputy defence minister warned that Tuesday's attack by Yemeni rebels on a major pipeline in the kingdom was "tightening the noose" around peace efforts.
The Saudi-led coalition, which has been battling the Huthi rebels since March 2015, confirmed that its warplanes were carrying out multiple strikes across rebel-held territory.
"We have begun to launch air strikes targeting sites operated by the Huthi militia, including in Sanaa," a coalition official, who declined to be identified, told AFP.
A strike on one Sanaa neighbourhood killed at least six people and wounded 10, Dr Mokhtar Mohammed of the capital's Republic Hospital told AFP.
The coalition carried out 11 strikes on the capital in all, among 19 across rebel-held territory, the rebels' Al-Masirah television reported.
The raids began around 8 am (0500 GMT) while many Yemenis were asleep awaiting the end at sunset of the daytime fast observed by Muslims during the holy month of Ramadan, a witness told AFP
"There were many strikes," he added.
The rebels said their attack on the Saudi pipeline was a response to "crimes" committed by Riyadh during its bloody air war in Yemen, that has been criticised repeatedly by the United Nations and human rights groups.
The drone strikes further raised tensions in the region after the mysterious sabotage of several oil tankers and the US deployment of an aircraft carrier strike group and B-52 bombers to the Gulf over alleged threats from Iran.
'Tightening noose' on peace
Saudi Arabia's deputy defence minister, Khalid bin Salman, charged the pipeline attack was carried out on Iranian orders.
"The attack by the Iranian-backed Huthi militias against the two Aramco pumping stations proves that these militias are merely a tool that Iran's regime uses to implement its expansionist agenda in the region," the prince said on Twitter.
"The terrorist acts, ordered by the regime in Tehran, and carried out by the Huthis, are tightening the noose around the ongoing political efforts."
The Saudi state minister for foreign affairs, Adel al-Jubeir, charged that the Huthis were "sacrificing the need of the Yemeni people for the benefit of Iran".
Key ally the United Arab Emirates warned of reprisals.
"We will retaliate and we will retaliate hard when we see Huthis hitting civilian targets like what happened in Saudi Arabia," the UAE minister of state for foreign affairs, Anwar Gargash, said on Wednesday.
Saudi Arabia and its allies intervened in Yemen when President Abedrabbo Mansour Hadi fled into Saudi exile as the rebels closed in on his last refuge in Yemen's second city Aden after sweeping through most of the rest of the country.
The intervention has retaken much of the south but the capital and most of the populous central highlands remain in rebel hands.
A grinding war of attrition has set in with third city Taez and the vital Red Sea aid port of Hodeida turned into battlegrounds.
In December, UN mediators brokered hard-won truce deals for both cities during talks in Sweden but the hoped-for momentum for talks on a comprehensive peace has failed to materialise.
On Tuesday, UN observers confirmed that rebel fighters had pulled out of Hodeida port and two other Red Sea terminals, unilaterally carrying out a key redeployment that was supposed to follow the December ceasefire.
UN envoy Martin Griffiths welcomed the pullback but warned the Security Council on Wednesday that the risks of a slide into all-out war remained high.
"There are signs of hope," he said, but there are also "alarming signs" of war.
Griffiths nonetheless hailed "a new beginning in Hodeida," where rebel fighters handed control of the port to coastguards, saying that "change is now a reality."
Hodeida is the main entry point for the bulk of Yemen's imports and humanitarian aid, providing a lifeline to millions of people who are on the brink of famine.
More than four years of conflict has triggered what the UN describes as the world's worst humanitarian crisis, with 24.1 million -- more than two-thirds of the population -- in need of aid.
"We are at a tipping point," warned the head of UN children's agency UNICEF.
"If the war continues any longer, the country may move past the point of no return," Henrietta Fore said.
'We have begun to launch air strikes targeting sites operated by the Huthi militia, including in Sanaa,' a coalition official, who declined to be identified, said.