CAIRO (Web Desk) –  Bombs fired from territory held by Yemen’s Houthi militia into a Saudi Arabian border town killed a Pakistani national and also injured four others on Monday, Arab TV reported, as the rebel group stepped up an apparent retaliation for weeks of air strikes by a Saudi-led coalition.

Meanwhile, a Moroccan F-16 warplane that is part of the Saudi-led force carrying out air strikes in Yemen has gone missing, Morocco’s military confirmed on Monday, while Houthi rebels and Saudi forces traded heavy fire across the border.

The disappearance of the Moroccan jet and the intense artillery and rocket battles could imperil an agreed five-day humanitarian truce in Yemen’s civil war due to begin on Tuesday.

Backed by Washington, a Saudi-led coalition has been bombing from the air 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 Arabs and the rebels remain the dominant force in the impoverished and chaotic Yemen, raising concern for the security of Saudi Arabia, the world’s top oil exporter, next door.

Morocco is one of eight Arab states to have joined Saudi Arabia in the military intervention against Houthi advances 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-Maseera 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.

It provided no details and it was impossible to verify the claim as phone lines were down throughout the war-torn area.