Saudi Arabia's ambassador to the US announced the military operation at a Washington news conference about a half-hour after the bombing began. The strikes commenced at 7 p.m. (2300 GMT), he said.

"The operation is to defend and support the legitimate government of Yemen and prevent the radical Houthi movement from taking over the country," Adel al-Jubeir (pictured above) told reporters in Washington.

"We will do whatever it takes in order to protect the legitimate government of Yemen from falling and from facing any dangers from outside militia. We have a situation where you have a militia group that is now in control or can be in control of ballistic missiles, of heavy weapons, and of an air force."

Saudi-owned broadcaster al Arabiya TV said 100 warplanes and 150,000 troops would be contributed to the operation by Riyadh.

At the same time as the Saudi announcement came a joint statement to the same end from five countries - Saudi Arabia, Bahrain, Kuwait, Qatar and the United Arab Emirates (UAE).

The statement said the countries had "decided to answer the call of President Hadi to protect Yemen and his people from the aggression of the Shiite militia."

Sea and air blockade

Houthi sources said in a statement that Saudi jets were hitting a military base, known as al-Duleimi, in Sanaa. They said anti-aircraft missiles were fired in response.

The Saudi-led coalition was said to have warned foreign ships from attempting to dock at Yemeni ports and declared the country's airspace a "restricted zone."

The White House said late on Wednesday it was coordinating closely with Saudi Arabia and other regional allies about the action against the Houthi. The help provided, sanctioned by President Barack Obama, was said to include intelligence and logistical support.

Egypt has said it is providing "military and political support" for the mission. Meanwhile, the al-Arabiya television network said planes from Morocco, Jordan, Sudan, Kuwait - as well as Gulf states - would be sent to Saudi Arabia to support operations.

Conflicting reports on president's whereabouts

The AP news agency reported on Wednesday that President Abed Rabbo Mansour Hadi had left the country by boat as Shiites closed in after he had fled his presidential compound.

However, an aide to the president was reported to have said Hadi remained in Aden and was encouraged by the Saudi airstrikes. "The president is in high spirits and thanks Gulf countries, Egypt, Jordan and Sudan and all countries in the region," Mohammed Marem, director of Hadi's office told the Reuters news agency.

Yemen has become increasingly divided between a north dominated by Houthis and a south largely controlled by Hadi supporters.

Former President Ali Abdullah Saleh, who resigned in 2012 after protests, has been accused of backing the Houthi rebels in an effort to regain influence.

rc/ipj (AFP, AP, dpa, Reuters)