Residents reported heavy clashes between the Houthis and mainly Sunni tribal fighters in the south of the country, while the air campaign sought to stall a fresh offensive by the Shi'ite Muslim group on Aden from the east.

Riyadh's intervention, a surprise move from a conservative monarchy better known for flexing its muscle in oil markets than through military might, is planned to last a month but could extend for five or six, a Gulf diplomatic source said.

He said satellite imagery had shown in January that the Houthis had repositioned long-range Scud missiles in the north, close to the Saudi border and aimed at Saudi territory. A Yemeni official said Iran, which says it has not armed the Houthis, was providing parts for the missiles.