"The red shirts say that those soldiers were heading to Bangkok as reinforcement for a potential crackdown on their protests which have been running for five months now," he said.
"The army though says these soldiers were in fact heading towards the troubled southern provinces of Thailand as part of a regular rotation of soldiers."
Earlier on Wednesday, the Thai army said that it would use force to disperse anti-government protesters occupying Bangkok'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 announced they would not march to the city's Silom financial district but instead stay at their main protest camp "indefinitely".
Abhisit Vejjajiva, the prime minister, said on Wednesday that he is ready to negotiate with protesters but only when they agree to abide by the law.
Panitan Wattanyagorn, a government spokesman, said an attempt to negotiate is being hindered by illegal demonstrations in Bangkok.
For their part, the red shirts say they are willing to hold talks through a third party to avert bloody clashes with troops.
Speaking to Reuters news agency on Wednesday, two of the protest leaders said they would consider offers of dialogue, but not from the government.
"We believe a crackdown is coming before April 25 and we need to make a compromise," Kwanchai Praina, one leader, said.
"I will propose in a meeting later today that we consider house dissolution in three months."
However, Jatuporn Prompan, a core red-shirt leader, was quoted by the AFP news agency as saying that talks were "out of the question.
The red shirts have been reinforcing defences at their base, and prepared homemade weapons including hundreds of sharpened bamboo poles and broken up pavement slabs.
An attempt by security forces to disperse the red shirts on April 10 erupted into the worst political violence Thailand has seen in almost two decades, leaving 25 people dead and more than 800 wounded.
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.
The unrest has taken a toll on the city's patience. Some residents on Tuesday evening tried to chase red-shirt protesters out of their camps.
Shouting "Kill them, kill them", several residents scuffled with a man believed to be a red shirt protester.
The red shirts consist mainly of poor rural workers pro-democracy activists who opposed the military coup that ousted the then prime minister, Thaksin Shinawatra, in 2006. They want parliament dissolved immediately and new elections called.
They believe Abhisit's government is illegitimate because it came to power through a parliamentary vote after disputed court rulings ousted two elected, pro-Thaksin administrations.