The UN secretary general has called on both sides in Thailand's growing political crisis to return to dialogue and avoid any steps that could risk further violence.

With deadly street clashes breaking out between security forces and anti-government red shirt protesters in Bangkok, Ban Ki-moon said on Friday he was monitoring the situation in Thailand with "growing concern".

"He appeals to both the protesters and the Thai authorities to do all in their power to avoid further violence and loss of life," Ban's press office said in a statement.

"He strongly encourages them to urgently return to dialogue in order to de-escalate the situation and resolve matters peacefully."

The statement released at UN headquarters in New York came after Thai troops opened fire on protesters as the military imposed a lockdown in central Bangkok in a bid to end more than two months of red shirt street protests.

At least 10 people were reported to have been killed and more than 100 wounded.

The clashes came after Abhisit Vejjajiva, the Thai prime minister, shelved a plan for November elections.

Echoing Ban's comments, US officials also urged all sides in the dispute to show restraint and resolve their differences peacefully.

"We are deeply concerned about the situation on the ground in Thailand," Philip Crowley, spokesman for the US state department, told reporters.

"We continue to strongly encourage everyone involved… to show restraint and to find a way to work peacefully through these differences and do so in a way that strengthens democratic institutions," Crowley said.

The US embassy in Bangkok has been closed since Thursday as a result of the rising tensions in the Thai capital.