The BBC's Chris Hogg reports on the failure to negotiate an end to the unrest
Thai officials have ruled out UN-backed mediation in an increasingly violent dispute with anti-government protesters on the streets of Bangkok.
A protest leader suggested the talks, but a government spokesman was quick to rebuff the offer, saying outside groups should not interfere.
At least 31 people have died since Thursday, when soldiers and police moved in to shift the demonstrators.
The protesters, known as red-shirts, have been camped in Bangkok for months.
They want Prime Minister Abhisit Vejjajiva to resign and call a new election.
A red-shirt leader, Nattawut Saikua, said protesters were willing to hold UN-moderated talks to end the stand-off, providing that the army withdrew from the area around the red-shirt camp.
But government spokesman Panitan Wattanayagorn insisted that no outside help was needed.
"We reject their demands for UN mediation... No Thai government has ever let anyone intervene with our internal affairs," he said.
Plea to Red Cross
A state of emergency has been declared in more than 20 provinces across the country - mostly in the protesters' northern heartlands - in a bid to stop more demonstrators heading to the capital.
14 Mar: Red-shirts converge on Bangkok, occupy government district
16 Mar: Protesters splash their own blood at Government House
30 Mar: Talks with government ends in deadlock
3 Apr: Occupy Bangkok shopping district
10 Apr: Troops try to clear protesters; 25 people are killed and hundreds injured
13-14 May: 16 killed in Bangkok clashes
15 May: Eight killed in street battles
Officials have urged women and older people to leave the protest zone by Monday afternoon, and have asked for help from the Red Cross.
About 5,000 people remain in the encampment, where food and water are running low amid a blockade on the area.
Sporadic clashes were reported on Sunday, but not on the same scale as previous days.
The BBC's Rachel Harvey says streets around the protest area are littered with fires; thick black smoke is rising into the air.
She says the pattern of demonstrations is that the protesters set up a disturbance and fires in one area, and the troops try to gain control of that area - then the demonstrators move somewhere else.
The latest fighting flared on Thursday, as the army and police moved to isolate a fortified protest camp.
Thousands of demonstrators remain behind makeshift barricades of rubber tyres, sandbags and bamboo stakes in the Ratchaprasong commercial district.
Mr Abhisit has declared Monday and Tuesday as public holidays and delayed the start of Bangkok's school term, but a planned curfew was cancelled.
On Saturday, Mr Abhisit said the army would not back down in its operation to clear the protesters.
"We cannot leave the country in a situation where people who don't obey the law are holding hostage the people of Bangkok, as well as the centre of the country," he said.
Several hundred protesters are gathering in other parts of Bangkok
Mr Abhisit and security officials have insisted that their operations against the protesters have targeted armed "terrorists" who they say have infiltrated the protesters.
Earlier, the army declared "live-fire zones" in some areas as it attempted to cut off the camp from supplies and reinforcements.
US-based Human Rights Watch warned that the zones had set the Thai authorities on a "slippery slope" towards serious human rights abuses.
More than 200 people have been injured since Thursday, and 27 people have been sent to jail, each given six-month sentences. All the fatalities have been civilians.
The clashes have raised questions about the stability of Thailand, South East Asia's second-largest economy.
Many of the protesters are from poor rural areas in northern Thailand where support is still strong for former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 coup.
Mr Thaksin has called on the government to withdraw troops and restart negotiations. He is living abroad to avoid a jail term on a corruption conviction.