Fighting between ethnic rebels and government troops rumbled on for a third day on Tuesday as activists warned that the violence, which has sparked a refugee exodus from Myanmar, could escalate in the aftermath of contentious elections.

Clashes at key points along the Thai-Myanmar border since Sunday have sent some 20,000 panicked villagers into Thailand, which already shelters a quarter-million ethnic minority refugees from brutal campaigns by the Myanmar army.

The exodus underlined Myanmar’s vulnerability to unrest following the country’s first election in two decades on Sunday, which was billed by the ruling junta as a key stage in its self-proclaimed road to democracy. Its political opponents and Western nations have decried the vote as unfair and repressive.

President Barack Obama said on Monday it was unacceptable for Myanmar’s government to “steal an election” and hold the people’s aspirations hostage to the regime’s greed and paranoia.

UN Secretary-General Ban Ki-moon said the voting was not inclusive enough and lacked transparency.

For a third day on Tuesday, sporadic gunfire erupted in Myawaddy. Refugees told Thai officials, however, that government forces had retaken the Myanmar border town and that the fighting was likely to end, according to Thai Gov Samard Loyfar of adjacent Tak province.

“We have to evaluate the situation to see if the clashes have actually ended before sending them back,” he said.

By Tuesday morning, some 20,000 refugees had fled into Thailand, said government spokesman Panitan Wattanayagorn. He said the government was working with humanitarian groups to tend to them and remained concerned about the violence escalating.

“At the moment, officers along the border have beefed up security, especially at the spots where clashes occurred,” Panitan said.

Myanmar has been ruled by the military near-continuously since 1962, and rebellions by its ethnic minorities predate its independence from Britain in 1948.

Sunday’s election was the first in Myanmar, also known as Burma, since a 1990 vote won by pro-democracy leader Aung San Suu Kyi’s party, which was barred from taking power and boycotted the new polls.

The regime says the election heralds a transition to civilian rule, but junta-backed candidates are virtually certain to dominate the new parliament.