Saudi-led forces resumed military operations in Yemen after a five-day ceasefire ended late on Sunday, and Yemen's foreign minister blamed the Iranian-backed Houthis for failure to renew the truce.

The end of the ceasefire came despite appeals by the United Nations and rights groups for extra time to allow the delivery of badly needed humanitarian supplies to the country of 25 million, one of the most impoverished in the Middle East.

‘That's what we said before - that if they start again, we will start again,’ Yemeni Foreign Minister Reyad Yassin Abdulla told Reuters.

He said the coalition was not considering any new ceasefire but would not target air and sea ports needed for aid shipments.

Saudi-led forces conducted three air strikes on Yemen's northern Saada province on Monday, according to Houthi media. The Saudi-owned al-Arabiya television reported heavy shelling by Saudi forces at Houthi outposts across the border after the fighters fired mortars at an army post in Saudi Arabia's southern Najran province.

Earlier in the day, residents said that warplanes struck the Houthi-held presidential palace in Yemen's southern port of Aden as well as groups of militiamen on the western and eastern approaches to the city and the international airport where Houthis and local fighters have been clashing.

In the capital Sanaa, residents said Houthi anti-aircraft guns opened fire on Arab jets on Monday morning but there were no air strikes there.

There was no word on casualties in any of the incidents.

Saudi Arabia and its Sunni Muslim allies have been conducting an offensive against the Houthis and units loyal to former president Ali Abdullah Saleh for more than six weeks, saying the rebels are backed by Shi'ite Muslim power Iran.

The campaign has yet to reverse the Houthis' advance into Aden and along battlefronts across Yemen's south.  

US Secretary of State John Kerry said on Monday that Washington supported extending the truce, but that maneuvers by the Houthis made that difficult.

‘We know that the Houthis were engaged in moving some missile-launching capacity to the border (with Saudi Arabia) and, under the rules of engagement, it was always understood that if there were proactive moves by one side or another, then that would be in violation of the ceasefire arrangement,’ he said.

STRIKES TO AVOID AID ROUTES

Iran's foreign minister on Monday called on the United Nations to take on a more active role in Yemen, including establishing a presence on the ground to ensure that humanitarian aid could be distributed.

‘We believe the U.N. needs to create a protected zone in Yemen to receive humanitarian aid... it is time for the U.N. to take control of the situation,’ Mohammed Javad Zarif said through an interpreter in a televised news conference.

Two Iranian warships have begun escorting an Iranian cargo ship off Yemen's waters, the vessel's captain said in remarks published by Iran's Tasnim news agency on Monday.

The vessel is reportedly carrying aid to the Houthi-controlled port of Hodaida, but the Saudi-led coalition has imposed an arms embargo on Yemen's ports and air space and will likely block its path before its scheduled arrival on 21 May.

The United Nations special envoy to Yemen Ismail Ould Cheikh Ahmed had called on Sunday for the five-day ceasefire to be extended during a meeting of Yemeni parties in the capital Riyadh which the Houthis did not attend.

A senior adviser to Iran's supreme leader Ali Khamenei said on Monday Saudi Arabia was an inappropriate venue for Yemeni reconciliation talks because it was not neutral.

Saudi-led forces, which have been conducting air strikes on Yemeni since 26 March, have come under criticism after they targeted the runways of Sanaa and Hodaida airports.

The Yemeni foreign minister said resumed coalition air strikes would spare facilities needed to deliver aid supplies.

‘They will keep places for aid to come. They will keep places safe like Sanaa airport, Hodaida seaport, Aden seaport. We will encourage and support any humanitarian aid to come in,’ he said.

Austrian energy group OMV, which operates from Yemen's central Masila oil field, on Monday declared force majeure for the blocks it operates in the country effective 23 April.

The company cited security reasons for the move, after tribes believed to be linked with Yemen's al Qaeda branch seized much of the oil-rich area in Hadramout province last month.

Modest oil and gas exports have provided the bulk of GDP and threats to the industry add to the impoverished country's woes.

UN agency OCHA said on Saturday that 1,820 people have died in Yemen's conflict since 19 March, and 7,330 have been wounded and over half a million have been displaced.