By Mohammed Ghobari and Mohammed Mukhashaf

CAIRO/ADEN (Reuters) - Jets from a Saudi-led alliance destroyed the runway of Yemen's Sanaa airport on Tuesday to prevent an Iranian plane from landing there, Saudi Arabia said, as fighting across the country killed at least 30 people.

Yemeni Vice President Khaled Bahah had called on the Houthis on Monday to heed a U.N. Security Council demand for an end to fighting, which the Red Cross says has pushed Yemen into a humanitarian catastrophe.

Houthis seized the capital Sanaa last September, demanding a more inclusive government and crackdown on graft. Talks with President Abd-Rabbu Mansour Hadi collapsed and he fled into exile. Chaos set in as the Iran-allied Houthi forces swept southwards, fighting loyalist army units, regional tribes and al Qaeda militants.

Top world oil exporter Saudi Arabia, rattled by what it sees as expanding Iranian influence in the region, has been leading a Gulf Arab coalition in waging air strikes on Houthi targets since late March. Riyadh said the campaign moved to a new phase last week, but fighting has intensified again since Sunday.

In Sanaa, air force planes from the Saudi-led coalition bombed the runway of the country's main airport to stop an Iranian flight landing, officials from both sides said, in a move that will further complicate humanitarian efforts to fly urgently needed aid into Yemen.

Brigadier General Ahmed Asseri, spokesman for the Saudi-led coalition, told Reuters that the airport was bombed after an Iranian aircraft refused to coordinate with the coalition and the pilot ignored orders to turn back.

He said the bombing of the runway made it unusable for planned aid flights. Airport officials said the strikes set a civilian aircraft operated by Yemeni Felix Airways ablaze. A cargo plane was also hit they said.

An official at the Yemeni civil aviation authority said the runway was targeted by 20 sorties that destroyed both the take-off and landing runways.

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Iranian state news agency IRNA, confirming the incident, said the pilots had ignored "illegal" warnings from Saudi jets to turn back before the runway was bombed. The agency said the plane was carrying humanitarian aid to Sanaa.

The Houthis' al-Masirah television said the plane was scheduled to carry wounded victims of the Saudi-led strikes for treatment in Iran.

A civil aviation official said the airport at the Red Sea city of Hodeidah had also been bombed, but appeared to be still operational. Officials said aid flights would be diverted to Hodeidah until Sanaa airport is repaired.

HOUTHI LEADER'S HOME BOMBED

Earlier on Tuesday, Saudi-led jets bombed a private villa that nearby residents said belonged to Abdullah Yahya Hakim, a senior Houthi official who was among a number of officials blacklisted by the U.N. Security Council in November.

Local residents also reported heavy clashes overnight in oil-producing Marib province east of Sanaa, in the city of Taiz in central Yemen, and in the southern port city of Aden.

At least 15 people were killed in the district of Sirwah and around Marib city, the sources said, as tribesmen allied with Hadi tried to stop Houthis and troops loyal to former president Ali Abdullah Saleh from advancing on the provincial capital.

The Houthis say their advance on Marib is to flush out militants belonging to al Qaeda in the Arabian Peninsula (AQAP), one of the most active branches of the Sunni Muslim militant network and an enemy of the Shi'ite Muslim Houthis.

In the southern province of al-Dhalea, local militiamen said they had killed at least 13 fighters from the Houthi group and troops loyal to Saleh in two separate ambushes. Snipers in Aden also shot and killed two civilians, residents said.

As the civil war rages on and the impoverished country sinks deeper into a humanitarian emergency, Yemenis warn that it will get ever harder to restore credible central state authority, raising the risk to nearby oil shipping lanes.

VICE PRESIDENT PLEADS FOR NEGOTIATED SOLUTION

The fighting has doubled the number of people displaced by the violence from the previous estimate of 150,000 on April 17, the U.N. humanitarian agency OCHA said on Tuesday.

Speaking in Saudi Arabia on Monday, Vice President Bahah said Yemenis should seek a negotiated way out of the crisis based on a U.N. Security Council resolution passed in April.

The Houthis have rejected the resolution, which imposes an arms embargo on them and on Saleh's supporters, and calls on them to lay down their weapons and leave Yemen's cities.

"The brothers in Ansarullah are called on to fear God ... and to stop their war on the cities," Bahah said, according to Yemeni news website www.voice-yemen.com. Ansarullah is the group's official name.

Bahah is popular among many of Yemen's feuding parties, and his appointment earlier this month created some hope that a negotiated solution could be reached.

(Additional reporting by Angus McDowalla in Riyadh, Sam Wilkin in Dubai and Omar Fahmy in Cairo, Writing by Sami Aboudi; Editing by Mark Heinrich)