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Yemen's Houthi  rebels said Monday that they shot down a warplane of the Saudi-led coalition, shortly after Morocco said one of its fighter jets had gone missing on a sortie. 

"The air defence of the tribes shot down a warplane over Wadi Nushur, in Saada," the rebels' northern stronghold, Houthi news channel Al-Masirah said, broadcasting images of tribesmen celebrating around the wreckage of a plane. 

The television said it was an F-16 fighter jet while the footage showed a Moroccan flag on a broken rudder lying on a rocky patch.  

Morocco said contact was lost with the fighter and its pilot at 6 pm (1500 GMT) on Sunday, the official MAP news agency reported, citing an armed forces statement.  

"A second plane which was flying in formation was not able to see whether the pilot ejected," it added.  

A Saudi official said that an investigation was trying to determine the missing plane's approximate location.  

The development came a day ahead of the start of a proposed five-day humanitarian ceasefire.

The rebels as well as their allies in Yemen's splintered armed forces routinely fire anti-aircraft guns at warplanes launching strikes in the country since the Saudi-led campaign began March 26.

Yemen's state news agency SABA, which is held by the rebels, did not report on the plane's disappearance. Nor was there any mention of the aircraft in Yemeni media, or on the official website of former Yemeni President Ali Abdullah Saleh, whose forces back the Houthis. 

Morroco's state news agency MAP, citing a military statement, said the pilot of a second jet said he didn't see the pilot of the missing fighter eject. The military said it had launched an investigation into the plane's disappearance, without elaborating on a cause. 

Morocco has six F-16 jets stationed in the United Arab Emirates taking part in the Saudi-led coalition, which includes a group of other Arab countries.

The West say Iran backs the Houthis militarily, something both the Tehran and the rebels deny. 

The plane is the first to be reported missing over Yemen since the aerial campaign began on March 26, despite rumours in the early days of the campaign that a Sudanese plane had been downed, which later turned out to be false.

On Monday the Saudis also announced that Najran, a Saudi city on the border with Yemen, had been shelled by the Houthis, leaving one person dead.

Najran also came under attack last week.  

Sadah bombarded

The news of the apparently downed jet came as airstrikes on the Houthi stronghold of Sadah intensify.

The mountainous northern province, on the Yemeni border with Saudi Arabia, was hit for a third night early Monday, after the Saudi-led coalition declared the whole province a military target on Friday.

According to aid agencies, some 70,000 civilians have fled the province  which lived through six wars between 2004 and 2010.

Many more remain trapped and unable to flee the coalition air and artillery bombardment because of a lack of fuel for transport. 

"We are living under a very difficult and unprecedented humanitarian situation," said one Sadah resident, who asked not to be named.

"We want to leave Sadah but can't due to the financial situation and the shortage of fuel," the resident said. "The Houthis are also trying to stop civilians from fleeing." 

Riyadh has offered a five-day truce from 2000 GMT Tuesday to enable aid deliveries. It has stressed the offer is conditional on the rebels reciprocating and not exploiting it for military advantage.  

The raging conflict in Yemen has killed over 1,400 people, many of them civilians, since March 19, according to the United Nations.

The cease-fire, scheduled to begin Tuesday, would help ease the suffering of civilians in the Arab world's poorest country who increasingly lack food, fuel and medicine since the bombing campaign began. 

However, all sides in the conflict have warned they will resume hostilities if the cease-fire is broken.