A U.S. aircraft carrier was headed to the Arabian Sea Tuesday as Washington said it was monitoring Iranian vessels suspected of carrying weapons to Huthi rebels in Yemen in violation of a U.N. embargo.

In Geneva meanwhile the U.N. health agency said more than 900 people had been killed in Yemen since late March, when a Saudi-led coalition launched air strikes against Iran-backed rebels clashing with pro-government forces.

Saudi-led air strikes on a missile depot in Yemen's rebel-held capital Monday sparked explosions that left at least 18 people dead and 300 wounded, flattening houses and shaking faraway neighborhoods.

Many more people were feared to have been killed after two strikes hit the hilltop depot, leaving a trail of destruction in the Fajj Attan area of Sanaa which was covered in thick clouds of smoke.

Refugees from war-torn Yemen describe the terror of intense airstrikes as they arrive in the Horn of Africa, where aid agencies are fearing an influx of people.

On the sun-blasted shores of Djibouti, those who have taken a perilous boat ride across the Gulf of Aden describe the horror of the airstrikes that pounded their homes in Yemen.

A Red Cross plane loaded with medical aid landed in Sanaa on Saturday, the second successive day of such deliveries to the capital of conflict-hit Yemen, a spokeswoman said.

The aircraft was carrying supplies the International Committee of the Red Cross says are urgently needed to treat the wounded after intense fighting between Shiite rebels and loyalists of President Abedrabbo Mansour Hadi.

The United Nations called Friday for an immediate "humanitarian pause" of at least a few hours each day in Yemen to allow desperately needed aid to enter the conflict-ravaged country.

The Red Cross and U.N.'s children agency finally managed Friday to get two planeloads of medical aid into the country -- the first to reach Sanaa since Saudi-led air strikes against Shiite rebels began last month.

A Norwegian freelance journalist has been arrested in the Yemeni capital Sanaa and accused spying, media reports said on Thursday.

Raymond Lidal, 28, was detained in late March when he failed to show a journalist's visa while filming Saudi-led airstrikes against Huthi Shiite rebels who have seized the city, according to the Verdens Gang newspaper (VG).

Regional rivals Saudi Arabia and Iran traded accusations Tuesday over the escalating conflict in Yemen, which the U.N. rights chief warned was on the brink of "total collapse."

Aid groups warned of a humanitarian crisis unfolding with air and sea blockades making it impossible to send desperately needed assistance as casualties mount.

Saudi-led warplanes bombed Yemen's main international airport and a renegade troop base in the capital Sunday, as Arab leaders vowed to pummel Iranian-backed rebels until they surrender.

The raids on the country's main airport came just hours after U.N. workers were evacuated following deadly fighting that has sent tensions soaring between Tehran and other Middle East powers.

At least 200 U.N. staff and other expatriates were evacuated from the Yemeni capital on Saturday, as an Arab coalition pressed a bombing campaign against Shiite rebels in the city, aid workers said.

"More than 200 staff of the United Nations and foreign embassies and firms left through Sanaa airport on Saturday afternoon," one aid worker told AFP, without specifying their destination.

Hundreds of terrified civilians fled the Yemeni capital on Thursday after a sleepless night of heavy Saudi air raids against Shiite Huthi rebels.

"I'm leaving with my family -- Sanaa is no longer safe," said Mohammed, loading personal belongings onto a minibus in a northern district of Sanaa, which has been under Huthi control since September.