Yemen's Houthis have responded to Saudi-led air strikes against Yemen's capital Sana'a by launching rockets across the border with the Arab kingdom, as the spectre of another sectarian war looms in the region.
Saudi Arabia launched the aerial bombings against the Shi'ite Muslim group which control Sana'a's airport and a military base.
The military operation, called "Decisive Storm", is backed by the other four Gulf Arab countries: UAE, Kuwait, Bahrain and Qatar and aims at protecting Yemen against the Iranian-backed Houthi militia. Jordanian air force fighter jets are also participating in the Saudi raids.
"This is in line with supporting legitimacy in Yemen and its security and stability ... Yemen and the Gulf's security is a high strategic interest (for Jordan)," an official Jordanian source told Reuters.
Morocco and Sudan also provided war planes to the operation according to Reuters.
At least 13 civilians were killed in the air strikes, according to AFP. After the start of the operation, pro-government forces seized back the international airport in Yemen's Aden.
Egypt, another key ally of the Gulf Arab countries, is providing political and military support to Saudi Arabia. The announcement by Egypt "stems from its historic responsibility towards the national Arab security" and "in defense of Yemen's security and stability," the foreign ministry said.
The air strikes came after Yemeni president Abd-Rabbu Hadi pleaded for help when the Houthis closed in on his government in exile in Aden. AP reported that Hadi fled the country in two boats from the port of Aden - but the news was dismissed by Yemeni government sources.
Mohammed Marem, director of Hadi's office, said the president was in "high spirits" after Arab countries launched the operation. "This operation has restored people's determination" to fight the Houthis, he said.
The US had set up a "joint planning cell" with Saudi Arabia to provide logistical and intelligence support, according to a spokeswoman for the National Security Council.
The Houthis, backed by Iran, have been advancing south from their base in the Yemeni capital for a week, taking an important military base to the north of the city on Wednesday.
The advance has prompted Saudi Arabia to mass troops on its porous border, with 100 fighter jets, 150,000 troops and navy units deployed in the military effort.
The Royal Saudi Air Force were also in control of Yemen's airspace.
It has also prompted scenes of chaos in Aden, residents said Wednesday, as diplomatic missions fled and residents of the largely Sunni city prepared for a Houthi invasion.
The Houthis have spread fast from their traditional tribesland in northern Yemen, taking Sana'a in January this year.