UPDATED 3:42pm: A coalition of regional allies including Saudi Arabia have begun a military operation to protect Yemen's government by launching air strikes against Huthi rebels.

"The operation is to defend and support the legitimate government of Yemen and prevent the radical Huthi movement from taking over the country," Saudi Arabia's ambassador to the United States Adel al-Jubeir told reporters in Washington.

Al-Jubeir said the coalition involved the countries of an existing alliance, the Gulf Co-operation Council, but that it had been joined by "outside countries".

"We have a coalition of over 10 countries," he said.

Yemen has been in turmoil since Huthi rebels launched a takeover bid in capital Sanaa last month.

They are now fighting in the port city of Aden.

The strife has raised fears Yemen could be torn apart by a proxy war between Iran, accused of backing rebels who follow a strain of Shi'ite Islam, and Sunni-ruled Saudi Arabia.

Yemen's acting foreign minister Riyad Yassin warned on Wednesday that the fall of Aden would mean the "start of civil war" as he drummed up Arab military support for President Abedrabbo Mansour Hadi, who was taken to a safe haven "within Aden" as the rebels closed in.

Separately, a statement issued in Riyadh in the name of Saudi Arabia, Qatar, Kuwait, Bahrain and the United Arab Emirates - the GCC countries without Yemen's neighbour Oman - said they had been asked for help by Hadi's embattled government.

Al-Jubeir said that for the moment the action was confined to air strikes on various targets around Yemen, but that other military assets were being mobilised and that the coalition "would do whatever it takes."