BANGKOK Thailand said on Friday it would only reopen a busy border crossing to Myanmar at the town of Mae Sai if it was assured its neighbor's soldiers would not cross the frontier in their battles against ethnic Shan guerrillas, Reuters reported. Tension flared between the two countries earlier this month, with dozens reported killed in border clashes between soldiers from the two sides, prompting Thailand to shut down the Mae Sai border checkpoint, 900km (560 miles) north of Bangkok and opposite the Myanmar town of Tachilek.