By Aziz El Yaakoubi and Mohammed Ghobari
RABAT/CAIRO (Reuters) - A Moroccan F-16 warplane has gone missing while on a mission with Saudi-led forces in Yemen, Morocco's military said on Monday, and Yemen's dominant Houthi militia said regional tribesmen shot down the aircraft.
The disappearance of the Moroccan jet and intensifying duels of heavy-weapons fire across the border between the Iran-allied Houthis and Saudi forces could endanger a five-day humanitarian truce due to start in Yemen on Tuesday.
Backed by Washington, the Saudi-led coalition has been bombing Houthi rebels and army units loyal to ex-president Ali Abdullah Saleh since March 26 with the aim of restoring exiled President Abd-Rabbu Mansour Hadi.
The Houthis' ties to regional rival Iran have rattled the Gulf Arab states and the rebels remain the dominant force in Yemen's civil war. Saudi Arabia, the world's top oil exporter, regards the Houthis' rise as a grave threat.
Morocco is one of eight Arab states to have joined Saudi Arabia in the military intervention against Houthi advances against Hadi supporters and has had F-16s stationed in the United Arab Emirates (UAE).
"One of the F-16s of the Royal Armed Force put at the disposal of the coalition led by Saudi Arabia to restore the legitimacy in Yemen went missing on Sunday at 6 p.m. local time," Morocco's military said in a statement.
The Houthis' official news channel al-Masirah said on Monday that anti-aircraft guns had downed an F-16 over in the remote Wadi Nashour area in the northwestern province of Saada, a Houthi stronghold bordering on Saudi Arabia.
The channel showed gun-toting tribesmen on a rocky hillside pumping their fists and chanting, "Death to America!" One man, holding a piece of what looked like aircraft wreckage, said: "God felled this plane. Even though our weapons are basic and modest, we'll shoot down all their planes, God willing."
There was no word on the fate of the pilots. A Yemeni Twitter (NYSE:TWTR) account published photos of what it described as the body of one of the pilots.
In the border conflict, the Houthis said they fired Katyusha rockets and mortars on the Saudi cities of Jizan and Najran near the frontier on Monday, after the Saudis hit Saada and Hajjah provinces in Yemen with more than 150 rockets.
A spokesman for the Najran civil defence department said that a school and house were hit and a Pakistani expatriate was killed and four people were wounded including a Saudi child.
Saudi-owned al-Arabiya television said Riyadh had deployed a "strike force" to its border with Yemen and showed a column of military trucks carrying tanks heading to the frontier.
Brigadier General Zayed al-Assiri said the force was distinguished by the variety of weapons it possessed and the large number of special forces and paratroopers.
Saudi planes also struck Houthi positions in the central city of Taiz and in the oil-producing province of Marib east of the capital Sanaa, which the Houthis captured last September.
DEVASTATION FROM ARTILLERY
Saudi-owned Ekhbariya TV showed Saudi buildings ripped open by apparent artillery shells but said there were no casualties. Houthi TV reported Saudi artillery and air strikes on civilian areas and said 13 people were killed.
At least 10 Saudi soldiers and border guards have been killed by shelling across the border. A Saudi jet crashed into the Red Sea after the start of the campaign in March and both pilots were rescued.
More than six weeks of air strikes by jets from the Sunni Muslim Gulf monarchies have failed to significantly push back the Shi'ite Muslim Houthis and militia and army units loyal to Saleh, who was forced from power by a popular uprising in 2011.
The Houthis accepted a five-day humanitarian ceasefire proposed by Saudi Arabia on Sunday but said they would respond to any violations of the pause.
Riyadh had said on Friday the truce could begin on Tuesday if the Iranian-allied militia agreed to the calm, which would let in badly needed food and medical supplies for civilians caught in zones of conflict.
A group of 17 international humanitarian groups working in Yemen said on Sunday that a five-day truce was not enough to provide sufficient relief to the large number of Yemenis affected by the crisis. They demanded a permanent ceasefire to halt a "rapidly deteriorating humanitarian crisis".
The relief agencies also condemned a Saudi warning to residents of Saada province on Friday to leave the area before it came under attack.
"Warning civilians does not exonerate the coalition from their obligation to protect civilians and civilian infrastructure, and we have seen in the last days that the warnings have not been enough to spare civilian lives," Save the Children's country director in Yemen, Edward Santiago, said.