The United States on Saturday called for restraint in deadly clashes between Thai protesters and security forces.

"We deplore this outbreak of political violence in Thailand, our long-term friend and ally, and urge good faith negotiations by the parties to resolve outstanding issues through peaceful means," White House National Mike Hammer said in a statement.

"The White House is closely monitoring the situation in Thailand and urges restraint by both protesters and security forces," he added.

Clashes occurred Saturday afternoon and evening as the Thai government tried to retake two major rally sites for the anti- government "red-shirts", who has been staging a rolling rally in Bangkok since March 12 demanding a House dissolution.

The situation seemed getting out of control as the "red-shirts" protesters and security forces started to exchange fire and throw grenades.

As many as 15 people died and more than 650 others were injured in recent clashes between the anti-government protesters and security forces in Thailand's capital Bangkok, according to the Bangkok Emergency Medical Service Center.