The worst fighting in years between Thailand and Cambodia spread on Tuesday to a third temple, as the neighbours exchanged artillery fire in border clashes that have killed 12 soldiers and displaced 50,000 villagers.

The fighting on Tuesday near the 11th century Preah Vihear temple followed unsuccessful international attempts to secure a ceasefire in the largely long-distance artillery battles that erupted on Friday at two other temples about 160 km to the west.

Preah Vihear was the scene of four days of deadly fighting in February and is the most prominent symbol of a border dispute between the Southeast Asian nations that goes back generations and has long stirred nationalist sentiment on both sides of the frontier.

The latest round of fighting began around the 1,000-year-old Ta Moan and Ta Krabey temples, which are on territory claimed by both countries. Since then, at least 12 soldiers on both sides have been killed and some 50,000 villagers forced to flee.

Each side has accused the other of starting the battles.

The U.S. threw its support on Monday behind efforts of Indonesia, current chair of the Association of Southeast Asian Nations, to mediate an end to the conflict.

Secretary of State Hillary Rodham Clinton said the U.S. was deeply concerned about the violence. In a statement, she urged both sides to exercise restraint and act to reduce tensions. Ms. Clinton said U.S. officials were talking with both countries.

Thailand has so far rejected outside intervention, saying the two countries have to resolve the dispute alone. But on Sunday, Thai Foreign Minister Kasit Piromya indicated that Bangkok might accept Indonesian military observers at the border, a proposal already accepted by Phnom Penh.

That would likely call for behind the scenes approval from Thailand’s powerful military, which so far has effectively vetoed the plan to bring Indonesian observers. Some have accused the Thai Army, which staged a coup in 2006 and continues to hold influence in domestic politics, of using the border dispute to raise its profile ahead of general elections expected by early July.

In recent years, political groups on both sides have accused their opponents of using the border issues to stir nationalistic fever and further their own domestic political agendas.

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