"The arrest of a Thai national will not lead to closure of the embassy [in Phnom Penh]. The Thai government will ensure security for the Cambodian Embassy in Thailand and we believe Cambodia will also take care of our embassy in that country," Suthep said.

"If bilateral relations become more violent, the government is ready to evacuate Thai citizens from Cambodia immediately," he added.

In early 2003, the Thai Embassy in Phnom Penh was burnt by rioters and several Thai-owned businesses in Cambodia were attacked following a rumour that a Thai actress had claimed the Angkor Wat temple - Cambodia's prized cultural icon - belonged to Thailand. An evacuation of Thai citizens followed the riots.

Suthep said the government had provided legal assistance for a Thai engineer arrested in Cambodia last week on charges of spying.

Siwarak Chothipong, 31, who works at Cambodia Air Traffic Service, is accused of supplying the Thai Embassy with details of ex-premier Thaksin Shinawatra's flight schedule, according to Cambodian police.

Thailand submitted a request to visit the detained suspect, which was being considered by Cambodia's Interior Ministry, said officials from both countries.

"We have to see him, whatever happens," said Chavanond Intarakomalyasut, secretary to Thailand's foreign minister. "Thailand categorically denies all of the spy allegations."

There was no reply from the Cambodian authorities yesterday, he said, adding that it was probably because it was a holiday.

Suthep told journalists that flight information on Thaksin's journey to Cambodia was not a secret, as the Aviation Department and Aeronautical Radio of Thailand had been asked to allow his chartered jet to fly over Thai airspace. After learning that the plane had Thaksin on board, the government refused to allow it permission to pass through Thai airspace as he has been convicted and was also facing charges of threatening national security, Suthep said.

The deputy prime minister said Thailand would use this evidence to defend Siwarak, but the government would not intervene in Cambodia's judicial system. Initially, Samart Corp - Siwarak's employer - sent a lawyer to assist him.

In Singapore, Prime Minister Abhisit Vejjajiva said yesterday that the ongoing diplomatic spat between Thailand and Cambodia would not affect cooperation among Asean members, emphasising that the problem must be solved by the two countries.

Abhisit, who is attending the 17th Asia-Pacific Economic Cooperation forum meeting, told journalists that Asean Secretary-General Surin Pitsuwan had advised that Thailand and Cambodia should resolve the tension before the Asean leaders meet US President Barack Obama today.

He affirmed that Thailand, currently the Asean chair, would not raise the issue at the meeting.