The United States on Monday updated its warning for American citizens planning to travel to Saudi Arabia, less than two weeks before President Barack Obama is due there.

Obama is to visit the kingdom on April 21 for a tense summit of Saudi Arabia and the other five Gulf Arab states at a time of crisis.

Obama has been criticized at home and in the Middle East for his reluctance to take tough action against Syria's Bashar al-Assad, allowing Russia to seize the initiative.

Saudi Arabia also faces domestic threats -- and spillover from its war in neighboring Yemen -- as was reflected in the updated State Department travel warning.

"The Department of State urges US citizens to carefully consider the risks of traveling to Saudi Arabia," it said.

"There continue to be reports of threats against US citizens and other Westerners, as well as locations frequented by them."

The statement, which replaces an earlier warning issued in September last year, warns that both Al-Qaeda and the Islamic State group are planning attacks in the kingdom.

"Possible targets include housing compounds, hotels, restaurants, shopping areas, international schools and other facilities where Westerners congregate," it said.

"Multiple attacks on mosques, as well as places where members of the Shia-Muslim minority gather, have occurred in Saudi Arabia over the past year."

The warning says US officials are banned from going within 50 miles (80 kilometers) of the Yemen border and urges Americans to stick to hotels and housing compounds.