Fire and smoke billows from an army weapons depot after it was hit by an air strike in Yemen's capital Sanaa on May 11. (Mohamed Al-Sayaghi/Reuters)

Clashes between Yemen’s Houthi rebels and a Saudi-led alliance intensified Monday ahead of a planned humanitarian cease-fire, with coalition air raids pounding targets in Yemen’s capital and the Houthis claiming to have downed a Moroccan fighter jet in the north.

Also Monday, a Saudi-owned television network reported that the Saudi military was sending a “strike force” of tanks and artillery to the Yemeni border.

The fighting could jeopardize a five-day “humanitarian pause” set to begin Tuesday to allow greater aid shipments from the United Nations and other groups. Impoverished Yemen has been devastated by the Saudi-led air war, which started March 26 in a bid to restore exiled President Abed Rabbo Mansour Hadi to power.

On Monday, New York-based Human Rights Watch said an air, naval and land blockade by the coalition is “keeping out fuel needed for the Yemeni population’s survival, in violation of the laws of war.”

Videos shot in Sanaa appeared to show an arms depot that was hit by airstrikes. Yemeni security officials said Sanaa came under heavy air bombardment on Monday afternoon, with the primary target being weapons and ammunition depots on Noqom mountain on the city’s northeastern outskirts. (The Washington Post)

“It is unclear how much longer Yemen’s remaining hospitals have before the lights go out,” said Joe Stork, the group’s deputy Middle East and North Africa director. Yemen desperately needs fuel for hospital generators and for water supply pumps, Human Rights Watch said.

Saudi officials have warned that the cease-fire could be called off if rebel offensives continue. Houthi officials said they would adhere to the truce only if the coalition is “serious” about holding its fire.

Saudi Arabia and its allies claim that Iran is backing the Houthis, a Shiite group. Tehran denies any direct links but has repeatedly denounced the Saudi-led attacks.

Six weeks of intense air attacks, however, have failed to significantly loosen the rebels’ hold in key areas such as Sanaa, the capital. Airstrikes Monday targeted suspected weapons depots on the outskirts of the city, local media reported.

Residents posted pictures on social media of the aftermath of the strikes, which they said set off secondary explosions. Local media also reported casualties, but the reports could not be independently verified.

The United Nations’ humanitarian coordinator for Yemen, Johannes van der Klaauw, said Sunday that coalition strikes over the weekend had targeted schools and hospitals, in breach of international law.

Meanwhile, Houthi fighters claimed Monday that they downed a Moroccan F-16 warplane taking part in the Saudi-led airstrikes.

A rebel-run television channel, al-Masirah, said tribesmen loyal to the Houthis shot down the jet in Saada province, near the Saudi border. There was no word on the fate of the pilot.

The rebel claims could not be independently verified, but Moroccan state media reported that the F-16 went missing Sunday.

A private news Web site in Morocco reported that the jet was hit by fire from antiaircraft batteries during a reconnaissance mission along the mountainous border region between Saudi Arabia and Yemen.

The French-language Web site Le360 reported that the F-16 was part of a two-plane patrol that came under fire from gunners in mountain ranges rising to nearly 6,000 feet.

The report said both jets tried to evade the fire but that “it was too late.”

“One of the craft was hit and went into a spin,” Le360 reported, without giving a specific source for the account.

Morocco has six F-16 jets stationed in the United Arab Emirates as part of the coalition’s mission.

If the rebel report is confirmed, it could suggest increased antiaircraft capability and prompt a strategy shift in the airstrike campaign.

The Moroccan F-16 was the second coalition warplane to go down since the Saudi-led campaign began.

In late March, a Saudi fighter jet crashed off Yemen’s southern coast, but the pilot and co-pilot were picked up by a nearby navy vessel. Technical problems were blamed.

President Obama is scheduled to meet later this week with Saudi Arabia’s crown prince, Mohammed bin Nayef, as part of talks with Persian Gulf Arab envoys at Camp David, Md., over issues such as nuclear negotiations with Iran and the Yemen conflict.

Saudi Arabia’s King Salman had indicated that he would attend the gathering, but the country’s foreign minister said Sunday that the monarch would remain in Riyadh because of the cease-fire plans.

In northern Yemen, along the rugged border, rebels and Saudi forces exchanged fire. On the Saudi side, a civil defense official in the city of Najran said at least one person was killed and four injured in mortar and rocket fire, the Reuters news agency reported.

Murphy reported from Washington.

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