The Thai government says it will accept a ceasefire offer from a Red Shirt protest leader, if their fighters leave the street and return to their main camp in central Bangkok.

A Red Shirt leader reached out to Thai Prime Minister Abhisit Vejjajiva to propose the ceasefire, hours after the government ordered thousands of protesters to vacate an area in downtown Bangkok.

Korbsak Sabhavasu, an aide to the Thai prime minister, said he received a cellphone call from a Red Shirt leader named Nattawut Saikua late Monday. They talked for about five minutes.

But he said the two sides remained far apart.

"If he (Nattawut) is serious about solving the problem he is capable of doing it. He can simply call back his people. ... Once he calls them back, who would the troops shoot at?" Korbsak told reporters.

Nattawut's response to the ultimatum is unknown.

Hours earlier, the Thai government warned thousands of Red Shirt protesters to leave the barricaded protest zone by 3 p.m. local time, to avoid facing criminal charges and possible jail time.

"Immediately vacate the area that is considered dangerous," the government said during a televised announcement. "Terrorists are trying to cause deaths in the area."

The government offered free transport for citizens to return home.

Red Shirt reaction

But the deadline came and went with few Red Shirts complying with the government's demand that they vacate a three-square-kilometre area where they have been holed up for weeks.

There was also some confusion about how much the Red Shirt protesters knew about the ultimatum, as government leaflets were dropped far from the protest zone and electronic TV and cellular signals were blocked.

Earlier Monday, Nattawut said the Red Shirts wanted to negotiate and would submit themselves to the Thai courts.

"It's time to return peace to the country. We are ready to move toward peace and the negotiations," Nattawut said. "The more the situation goes on, the longer people's lives will be in danger."

Another Red Shirt leader, Jatuporn Prompan, called on the Thai monarch -- the 82-year-old King Bhumibol Adulyadej -- to intervene as he has done during prior political crises.

The government said Red Shirt protesters had to stop attacking members of the Thai military.

"The protest has to stop. Clashes have to stop. Activities that threaten security of the general public have to stop," government spokesman Panitan Wattanayagorn said.

"We have no policy to attack civilians. Our officers respond militarily when they are attacked," he said.

Clashes between protesters and Thai troops have killed 37 people in Bangkok over the past five days, as the military has surrounded the Red Shirts and limited their options.

Those clashes have taken their toll on the protesters -- their numbers within the protest area appear to have dwindled to 3,000 from at least 10,000 a week ago.

Still, the military operation to quash the Red Shirts outright has so far failed. Despite government rhetoric, it seems there are too few soldiers to accomplish the task. The soldiers are without armoured vehicles and many are poorly disciplined.

Both the police and the army are also believed to harbour so-called "watermelons," meaning officers who wear green uniforms but secretly support the Red Shirts.

"Despite all the violence so far, it is still unclear if the army would be willing to launch a full-on assault to break up the main protest site," said Andrew Walker, a Thailand expert at The Australian National University.

On Monday, protesters stole a fuel tanker from a gas station and tried to light it on fire, but were deterred by troops. Explosions and gunfire could be heard in and around the protest zone throughout the day.

The Red Shirts were also dealing with the loss of their purported military strategist, Maj. Gen. Khattiya Sawasdiphol. The rogue Thai general died on Monday, days after he was shot while speaking to reporters in Bangkok.

The Thai government says 66 people have died in the protests that began two months ago. Some 1,600 others have been injured.

The Red Shirts seek to oust Thai Prime Minister Abhisit Vejjajvia from office, claiming that he unjustly took power by manipulating the courts and relying on military support.

They support Thaksin Shinawatra, a billionaire who previously served as Thai prime minister until leaving his country to live in exile.

It is suspected that Thaksin has helped bankroll the ongoing protests.

With files from The Associated Press