Unidentified warplanes had earlier launched airstrikes on the main airport in the Yemeni capital, Sanaa, and its al Dulaimi military airbase, residents said.  That came soon after Saudi Arabia's ambassador in Washington, Adel al-Jubeir, announced the operation.  "We will do whatever it takes in order to protect the legitimate government of Yemen from falling," Jubeir told a news conference.  Yemen's slide towards civil war has made it a crucial front in mostly Sunni Saudi Arabia's rivalry with Shi'ite Iran, which Riyadh accuses of stirring up sectarian strife throughout the region and in Yemen with its support for the Houthis.  Read MoreWhy Yemen war may worry Gulf more than low oil prices  The crisis now risks spiraling into a proxy war with Iran backing the Houthis, and Saudi Arabia and the other regional Sunni Muslim monarchies supporting Yemeni President Abd-Rabbu Mansour Hadi.  Fighting has spread across the Arabian peninsula country since last September, when the Houthis seized Sanaa and advanced into Sunni Muslim areas, forcing Hadi out of the capital.  Jubeir said the assaults had been launched in response to a direct request by Hadi, who supported Washington's campaign of deadly drone strikes on a powerful al Qaeda branch based in Yemen. He has been holed up in Aden with loyalist forces since he fled Sanaa in February.  Hadi remains in his base in Aden and was "in high spirits",one of his aides said after the operation began.  A senior leader of Yemen's Houthi movement said the Saudi airstrikes amounted to aggression against his country and warned they would set off a "wide war" in the region.
Saudi Arabia and Gulf region allies launch military operations including airstrikes in Yemen to counter Iran-allied forces besieging Aden.