A US aircraft carrier was headed to the Arabian Sea Tuesday as Washington said it was monitoring Iranian vessels suspected of carrying weapons to Yemeni rebels in violation of a UN embargo.
Iran's regional rival Saudi Arabia, leading the air war on the rebels, meanwhile mobilised its National Guard for operations in Yemen, state media said, without clarifying how the 100,000-strong force would join the campaign.
In Geneva, the UN health agency said more than 900 people had been killed in Yemen since late March, when a Saudi-led coalition launched air strikes against the Iran-backed Huthi Shiite rebels clashing with pro-government forces.
And the International Organization for Migration announced a temporary suspension of its evacuation efforts due to insecurity.
Amid reports of a nine-ship Iranian convoy in the area, the US Navy said it was sending the USS Theodore Roosevelt and guided-missile cruiser USS Normandy "to ensure the vital shipping lanes in the region remain open and safe".
The deployment brings to nine the number of US warships in the area, but the Pentagon denied reports they had orders to intercept the Iranian vessels.
Strategically located on key shipping routes and bordering oil-rich Saudi Arabia, Yemen was plunged into chaos last year when the Huthis seized the capital Sanaa.
The coalition of Sunni Arab nations led by Saudi Arabia launched an air campaign against the rebels last month, vowing to restore the authority of President Abedrabbo Mansour Hadi, who fled to Riyadh as the rebels advanced on his southern refuge Aden.
The United States says it is not taking part directly in the strikes, but is providing intelligence and logistical support.
Coalition warplanes pressed their air strikes against the rebels and their allies in the security forces overnight, as the civilian death toll from a Monday raid on a missile depot in the capital rose to 38.
A further 532 people were wounded when the twin strikes sparked powerful explosions that flattened nearby houses, medics said.
The base was held by the elite Republican Guard, which remains loyal to former president Ali Abdullah Saleh who has been accused of siding with the Huthis in their fight against Hadi's forces.
Rebel spokesman Mohammed Abdulsalam slammed the strikes on the base as a "barbaric crime", insisting on Facebook that the "aggression will only unite the Yemeni people".
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- Nearly 3,500 wounded -
The coalition says it has carried out more than 2,000 strikes since the start of the campaign, gaining complete control of Yemeni airspace and knocking out rebel infrastructure.
In the central province of Ibb, several civilians were killed in a strike targeting rebel air defence missiles, placed in a residential area, witnesses said.
Farther south in Shabwa province, tribal sources reported several deaths in air raids and fighting.
The World Health Organization said the violence has left 944 people dead and 3,487 wounded as of April 17, based on numbers from health facilities in Yemen.
But the true numbers were likely higher since many people were not making it to hospitals for treatment.
IOM spokesman Joel Millman meanwhile said the migration agency had been forced to suspend its operations to evacuate foreigners "until further notice" due to "escalating difficulties faced in undertaking flight operations in recent days".
Any chance of a swift diplomatic solution faded as Saudi ambassador Abdallah al-Mouallimi told UN chief Ban Ki-moon "certain conditions" must be met for the air campaign to be suspended.
He said those were clearly spelt out in a resolution adopted by the UN Security Council last week which imposed the arms embargo on the rebels.
The resolution demands the Huthis pull back from all the territory they have seized, including from Sanaa, and that they return to peace talks.
Ban has called for an "immediate ceasefire" and time for a "passage to real peace".
Yemen has long struggled with deep tribal divisions and an insurgency by Al-Qaeda in the Arabian Peninsula (AQAP), considered by Washington as the jihadist network's most dangerous branch.
Al-Qaeda has taken advantage of the chaos to seize swathes of territory in Hadramawt province in the southeast, including its capital Mukalla.