Pictures circulated on Saturday also showed army snipers taking positions in buildings in the Rajprasong business district.
'Defending the barricades'
Al Jazeera's Aela Callan, reporting from Bangkok, said television footage appeared to show troops were firing indiscriminately, with reporters covering the protests being shot at in some incidents.
"It's a very confusing situation," she said. "What we're going to have to watch for now is the reaction of the red shirts."
She said some red shirt sources in the northeast of Thailand had told her they were being prevented from sending bus loads of supporters to join the protests in Bangkok, while other more "hardline" red shirts were looking at more creative ways to bring reinforcements into the city.
"There are still thousands of people down there and there are still thousands that are willing to defend the barricades," she said.
Sean Boonpracong, a red shirt spokesman, called on the army to end its operations against the protest camp saying the two sides were unmatched.
"We want the army to cease fire," he told Al Jazeera.
"We really seriously want peace. We are really concerned at what is taking place."
Other red shirt leaders have said they believe the death toll from the violence is much higher than the numbers confirmed killed.
The military said it would call in reinforcements to help it surround the protest site and would move into the encampment if the red shirts did not disperse.
"There is a plan to crack down on Ratchaprasong if the protest does not end," Sunsern Kaewkumnerd, the army spokesman, said.
"I cannot say how many troops are deployed because of security concerns, but there will be reinforcements to help troops seal the area and step up pressure on protesters."
|Protesters have fought back with burning tyres and petrol bombs [AFP]
With tensions rising, the UN secretary general, Ban Ki-moon, has called on all sides to avoid violence and resolve the crisis through talks.
"He strongly encourages them to urgently return to dialogue in order to de-escalate the situation and resolve matters peacefully," Ban's spokesman said in a written statement.
Abhisit Vejjajiva, Thailand's prime minister, had offered to call new elections if red shirts ended their two-month protest, but the demonstrators have insisted that the country's deputy prime minister must first be charged for the deaths of 25 red shirt supporters.
Panitan Wattanayagorn, a spokesman for the Thai government, told Al Jazeera on Saturday that discussions on holding new elections could only begin once the protesters disperse.
"When the protest ends, [the prime minister] will resume implementing the reconciliation plan and also consider the election after that. The election date was set on the 14 of November, but was rejected by the red shirt people," he said.
The red shirts say Abhisit's government is illegitimate because it came to power in a 2008 parliamentary vote following the controversial court-ordered dissolution of the previous pro-Thaksin government.