|Thai red shirt stand-off deepens|
On Wednesday, red shirt protesters stopped a train carrying about 70 soldiers and military hardware in Khon Kaen province.
Despite negotiations between the area's deputy governor and red shirt leaders, the train was still blocked by around 200 protesters on Thursday, police said.
Al Jazeera's Wayne Hay, reporting from Bangkok, said military officials insist the train is headed for the south of the country to help assist in containing a separatist insurgency, and is not part of any effort to end the red shirt protests by force.
That assurance however does not seem to have carried weight with the red shirts who have said they will continue to block the train's path for at least the next five days.
Our correspondent said that according to some reports an estimated 10,000 soldiers were already on stand-by in the Thai capital.
A previous attempt to force an end to the protests earlier this month led to bloody clashes on the streets of Bangkok in which 25 people died and hundreds were injured.
On Thursday thousands of red shirts began a march through Bangkok to the United Nations headquarters in the city.
The protesters said the march was to call for international intervention to bring a peaceful end to the stand-off.
A day earlier the Thai army had warned that it would use force to disperse anti government protesters occupying the city's main shopping district.
Action would be taken "considering the safety and lives of the people, and only when there is reason to do so, including for self-defence and in extreme cases", the government's Centre for the Resolution of the Emergency Situation said.
The warning came after red-shirt leaders said they planned to dig in at their camp in Bangkok's Silom financial district and continue their protests "indefinitely".
Piles of sharpened bamboo sticks and broken paving stones have also been stockpiled, triggering fears of new confrontations with security forces who are threatening to use tear gas and live rounds if necessary.
The red shirts have been camped out on the streets of Bangkok since March 12, with the standoff causing widespread disruption, closing shopping malls, hotels and causing millions of dollars in losses for Thailand's vital tourism industry.
On Wednesday night, skirmishes between opposing groups of protesters broke out in central Bangkok, with gangs of government supporters throwing bottles at red shirts camps.
Red shirt protesters responded with bottles, rocks, and fireworks.
The red shirts consist mainly of poor rural workers and pro-democracy activists who opposed the military coup that ousted the then prime minister, Thaksin Shinawatra, in 2006.
The red shirts meanwhile are sticking to their demand for parliament to be dissolved immediately, according to local media.
A report in The Nation newspaper on Thursday said red shirt leaders were insisting on an immediate dissolution of parliament and had rejected an appeal by a national economic council to wait until a budget bill was passed in July.
Some leaders of the red shirts had previously suggested they might consider a three-month timeframe for Abhisit Vejjajiva, the prime minister, to dissolve parliament and call elections.
With tensions mounting, the US government weighed into the crisis on Wednesday urging both sides to seek a peaceful resolution to the standoff.
"We would continue to encourage both sides to work out their disagreements peacefully," Philip Crowley, a State Department spokesman, told reporters in Washington.
"We don't believe that violence in any shape or form is a solution to this political challenge."