Cambodian soldiers patrol Preah Vihear temple, near Thailand, amid a build-up of troops on both sides.

The security situation along the disputed border at Preah Vihear temple is "worsening," Prime Minister Hun Sen said in a letter to his Thai counterpart Thursday, as troop levels on both sides increased.

More than 400 Thai soldiers and 800 Cambodian soldiers are now deployed along the border, following the arrest of three Thai protesters at Preah Vihear temple and the alleged incursion of more than 40 Thai troops at a pagoda claimed by Cambodia, government spokesman Khieu Kanharith told reporters Thursday.

Hun Sen said in his letter to Prime Minister Samak Sundaravej Thursday the number of increased troops near Keo Sikha Kiri Svara pagoda, near Preah Vihear temple, meant "the situation is worsening."

"The deteriorating situation is very bad for the relations between our two countries, and, therefore, I would like to ask Your Excellency to take measures to ease the tensions and order the Thai troops to withdraw from the area," he wrote.

The border is disputed by both countries on the land surrounding the cliff-top temple, which was added to Unesco's World Heritage list on July 7 and is claimed by Cambodia.

The World Heritage inscription was cause for celebration in Cambodia, but it sparked protests in Thailand, leading to the resignation of Thailand's foreign minister and closure of the entrance to the temple on the Thai side.

As the leaders of the two nations communicated Thursday, the actual situation on the border was quiet, and troops there said they have been ordered not to fire on the Thais unless fired upon first.

Thailand and Cambodia have meanwhile agreed to a special meeting July 21 over tensions along the border, in "a spirit of neighborliness," according to a Thai Foreign Ministry statement.

"In the meantime, the armed forces of both sides are in close contact," the statement said.

Cambodia Defense Minister Tea Banh told VOA Khmer Wednesday the situation can be resolved through negotiation.