By Mohammed Ghobari and Mohammed Mukhashaf
CAIRO/ADEN (Reuters) - A Moroccan F-16 warplane has crashed while on a mission with Saudi Arabia-led forces in Yemen, a Saudi military spokesman said on Monday, and Yemen's dominant Houthi militia said tribesmen shot down the aircraft.
The loss of the Moroccan military jet and intensifying volleys 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 early on Tuesday.
Saudi-led air strikes hit military bases and weapons stores in the Houthi-controlled capital, Sanaa, on Monday evening, setting off huge blasts that residents said launched rockets into the air which then crashed back down.
"The violent explosions can be heard from anywhere in the city and we feel they could land on our heads. We're living a life of terror," said Ahmed Fawaz, a resident of Sanaa.
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 to power.
The Houthis' ties to regional rival Iran have rattled the Gulf Arab states and the rebels remain the strongest force in Yemen's civil war. Saudi Arabia, the world's top oil exporter, regards the rise of the Houthis as a grave threat.
Morocco is one of eight Arab states to have joined Saudi Arabia in the military intervention against the Houthi advance. The coalition is also receiving logistical support from the United States, Britain and France.
"Now there's confirmation that the plane fell and its location has been determined," Brigadier General Ahmed Asseri, spokesman for the Saudi-led coalition told Saudi-owned Arabiya TV on Tuesday.
"The fate of the pilot remains unknown, but from the platform of Al Arabiya we hold the Houthi militias and their allies responsible for the safety of the pilot," he added.
The Houthis' official news channel al-Masirah said on Monday that anti-aircraft guns had downed the F-16 over in the remote Wadi Nashour area in the northwestern province of Saada, a Houthi stronghold bordering 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."
A Yemeni Twitter account published photos of what it described as the body of a pilot.
In the border fighting, 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 Ekhbariya TV showed Saudi buildings ripped open by apparent artillery shells, but said there were no casualties.
Al-Arabiya television also said Riyadh had deployed a "strike force" to its border with Yemen and showed a column of military trucks carrying tanks bound for the frontier, in an apparent escalation of its preparedness for a border war.
Houthi TV, meanwhile, reported Saudi artillery and air strikes on civilian areas and said 13 people were killed.
More than six weeks of air strikes by jets from the Sunni Muslim monarchies of the Gulf 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.
At least 10 Saudi soldiers and border guards have been killed by shelling across the border.
On Monday evening, residents of the southern port of Mukalla said an apparent American drone strike killed four local leaders of al Qaeda, which has capitalized on the breakdown in state order to operate openly in the city and in parts of south Yemen.
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 Shi'ite 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".
(Reporting by Aziz El Yaakoubi in Morocco, Mohammed Ghobari in Cairo and Praveen Mennon in Kuala Lumpur; writing by Sami Aboudi and Noah Browning; editing by Mark Heinrich and G Crosse)