An unspecified number of Westerners and Saudis were killed when explosions ripped through three residential compounds in Riyadh on Monday night, according to a hospital official in the city.

"Dozens of people were wounded and they are being treated at some 10 hospitals," the official at the al-Hamadi Hospital told Reuters. "I am sure there are dead."

The blasts came on the eve of US Secretary of State Colin Powell's visit to Riyadh.

On May 1, the State Department had warned Americans living in the Arab nation that 'terrorist groups may be in the final phases of planning attacks against US interests in Saudi Arabia'.

The government said other US citizens in the country 'should evaluate their own security situations, and should consider departing Saudi Arabia'.

With inputs from agencies