Governments around the world warned their citizens to avoid Thailand's capital Bangkok as deadly political violence paralysed the city centre Friday.

From Washington to London and Canberra governments also joined the UN in urging all sides to show restraint as riot police faced off against thousands of anti-government "Red Shirts" behind heavily fortified barricades.

Tensions in the long-running political standoff rose after five grenade blasts hit the area on Thursday night, leaving one Thai woman dead and scores wounded, including foreigners.

"There is a strong possibility of renewed violent clashes in Bangkok between demonstrators and security forces," Australia's foreign affairs department said, as it confirmed that an Australian was among the wounded.

"We advise you to reconsider your need to travel to Thailand."

The United States also warned that more violence was possible.

"Due to escalating violence in central Bangkok, all US citizens should avoid non-essential travel to Bangkok," the State Department said, upgrading a travel alert issued two days earlier.

"The possibility of more such attacks cannot be ruled out."

The State Department also urged travellers to exercise caution in provincial areas, warning that protesters could spread to other parts of the kingdom if dispersed in the capital.

Britain, France and Austria urged their citizens to stay away from the demonstrations, while Finland said it would move its embassy from the area to temporary premises.

London warned against all but essential travel to the Thai capital due to the "increasingly volatile" situation on the streets.

French foreign ministry spokesman Bernard Valero said in a statement: "We advise our compatriots to exercise the utmost caution and in particular not to go to places where there are political gatherings."

He expressed concern over stern warnings by Thai army spokesman Sunsern Kaewkumnerd that force could be used in a crackdown against the anti-government protesters.

"We reiterate our call to the authorities and protesters to act responsibly," Valero said.

"It is more indispensable than ever for dialogue to prevail over confrontation."

Austria's foreign ministry also issued a travel warning, advising against any unnecessary trip to the Thai capital, citing a "high security risk for the greater Bangkok area".

The United Nations appealed to authorities and anti-government protesters to avoid further violence and settle their dispute through dialogue.

"We appeal to both the protesters and the Thai authorities to avoid further violence and loss of life and to work to resolve the situation peacefully through dialogue," UN spokesman Martin Nesirky told a press briefing.

"This is a moment that requires restraint on all sides."

Nesirky said UN chief Ban Ki-moon was "very concerned about the continuing standoff and tension in Thailand and the potential for this to escalate."

The Reds began mass anti-government rallies in Bangkok in mid-March that degenerated into fierce clashes with security forces on April 10, leaving 25 dead and more than 800 injured.

The Thai authorities say Thursday's grenade attack came from within the Red Shirts' sprawling encampment, which has been fortified with sharpened bamboo stakes and piles of car tyres.

The protesters are demanding immediate elections.

burs-lb/pch