Refugees fleeing across the border from Myanmar will be allowed into Thailand if the violence there escalates, Prime Minister Prayut Chan-o-cha said on Tuesday.

He was speaking after the Myanmar army's air strikes against the Karen National Union (KNU) last week sent ethnic Karen civilians fleeing across the border to Thailand.

Many of them, however, just crossed the river back and stayed on the bank, too afraid to go back home.

"Thailand will not turn them away," Gen Prayut said. "When they are in trouble, we will not deny them entry to our country. However, this does not mean we will declare our intention to allow them in with open arms.''

Asked about reports that some refugees were denied entry on the border, Gen Prayut said that it was necessary to enforce requirements for legal entry.

"If there is a war, that's another scenario. In the event the situation escalates, leading to deaths and injuries, the government will put in place measures to deal with an influx," he said.

"We have years of experience, with nine refugee centres sheltering more than 400,000 refugees,'' Gen Prayut said.

Gen Prayut said the government would send them back when the situation across the border was safe.

Thailand already hosts about 100,000 refugees from Myanmar at camps along the border.