(Adds Cambodia's petition U.N. Security Council)

By Nopporn Wong-Anan

KANTARALAK, Thailand, July 19 (Reuters) - Thailand and Cambodia sent troops and heavy guns on Saturday to their disputed border, where hundreds of soldiers faced off for a fifth day over an ancient Hindu temple.

Despite the military build-up, both sides said they were ready to negotiate an end to the stand-off.

The dispute has raised investor fears of a major confrontation. Thailand's main stock index has fallen more than 23 percent since anti-government street protests in Bangkok began in late May, and could drop further if border tensions get worse, analysts said.

The Preah Vihear temple, perched on a jungle-clad escarpment that forms a natural boundary, has been a source of tension since the International Court of Justice ruled in 1962 it belonged to Cambodia, a decision that still rankles Thais.

The listing of Preah Vihear as a World Heritage site this month triggered a political uproar in Thailand, stoked by groups opposed to Prime Minister Samak Sundaravej whom they accuse of being a proxy of former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 coup.

The diplomatic sparring between Bangkok and Phnom Penh has intensified ahead of high-level talks on Monday involving the two countries' defence ministers.

Thailand summoned Cambodia's ambassador on Friday to respond to Prime Minister Hun Sen's charge that Thai troops had "encroached" on Cambodian territory" and that the situation was "worsening".

In a letter to Hun Sen, Samak said Cambodian troops and buildings on the disputed 4.6 sq km (1.8 sq mile) area were a "violation of Thailand's sovereignty and territorial integrity".

He added his government was "resolved to seek a just and peaceful solution to the situation".

Cambodia has asked the United Nations' Security Council to discuss the border dispute with Thailand, Thai Government Spokesman Wichianchot Sukchotrat told Reuters.

"We have been informed by our ambassador to the U.N. that Cambodia has filed a complaint over the dispute to the U.N.," Wichianchot said.

NATIONALIST FERVOUR

In Cambodia, Preah Vihear has become a key issue in the run-up to next Sunday's election as Hun Sen's ruling party and the opposition vie for votes by stoking nationalist fervour.

"They should focus on issues like fighting poverty and corruption instead of using Preah Vihear for their political interests," said Kek Galabru, head of the LICAHDO rights group.

Lieutenant General Sujit Sitthiprapha, commander of Thailand's Second Army, said more troops were sent to the border after Cambodia reinforced its forces at the temple.

A Reuters witness saw a convoy of eight Thai army trucks ferry several hundred soldiers to the border. In another convoy, trucks towed heavy artillery.

"If things escalate, we can use those troops right away," Sujit said, although he added the soldiers at the temple "were still talking to each other".

Thailand estimates it is facing 1,200 Cambodian troops in and around the temple, although Phnom Penh disputes that figure.

Chea Mon, Cambodia's military commander at Preah Vihear, said the situation was calm at the temple where his men faced about 400 Thai soldiers.

"We are protecting our borders. We will leave it to government leaders to solve this issue," he said by telephone. (Additional reporting by Ek Madra in Phnom Penh; Writing by Darren Schuettler; Editing by Matthew Jones)

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