WASHINGTON - US President Barack Obama has called the kings of Saudi Arabia and Bahrain to express "deep concern" about violence in Bahrain and urged "maximum restraint," White House spokesman Jay Carney said on Wednesday.

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He told reporters at the White House that "the president expressed his deep concern over the violence in Bahrain and stressed the need for maximum restraint" in separate phone calls to Saudi King Abdullah and Bahraini King Hamad bin Isa al-Khalifa.

The calls came as Bahrain's Defense Forces on Wednesday imposed a curfew in several areas of the capital and moved against protesters with the backing of helicopters.

A day earlier, the Bahraini king declared a state of emergency in the tiny Gulf nation for three months to deal with the unrest and protests that swept the country recently.

The Gulf Cooperation Council, which groups Bahrain, Saudi Arabia, Kuwait, Qatar, the United Arab Emirates and Oman, sent troops on Monday to Bahrain to maintain order and security. Most of the troops are from Saudi Arabia.

Bahrain is home to the US Navy's 5th Fleet.