Myanmar aircraft carried out three strikes overnight on Sunday, according to Free Burma Rangers, a humanitarian relief agency that delivers medical and other assistance to villagers.

The strikes severely injured one child but caused no apparent fatalities, a member of the agency said.

Thai Prime Minister Prayuth Chan-ocha on Monday acknowledged the problems across his country's western border and said his government is preparing for a possible influx of people.

"We don't want to have mass migration into our territory, but we will consider human rights, too," Prayuth said.

Asked about people who have already fled into Thailand, Prayuth said, "We have prepared some places, but we don't want to talk about the preparation of refugee centres at the moment. We won't go that far."

About 2500 people, including 200 students, have crossed the Salween River into northern Thailand's Mae Hong Son province, according to Burma Free Rangers.

An estimated 10,000 people are believed to be displaced in Myanmar's northern Karen state, the agency said.

Video shot Sunday shows a group of villagers, including many young children, resting in a forest clearing inside Myanmar after fleeing their homes. They carried their possessions in bundles and baskets.

The bombings may have been retaliation against the Karen National Liberation Army for having attacked and captured a Myanmar government military outpost on Saturday morning. The group is fighting for greater autonomy for the Karen people.

Leaders of the resistance to last month's military coup that toppled Myanmar's elected government are seeking to have the Karen and other ethnic groups band together and join them as allies, which would add an armed element to their struggle.

The air strikes mark an escalation in the increasingly violent crackdown by the Myanmar government against opponents of the February 1 military coup.

At least 114 people across the country were killed by security forces on Saturday alone, including several children - a toll that prompted a UN human rights expert to accuse the junta of committing "mass murder" and criticise the international community for not doing enough to stop it.

The coup, which ousted the government led by Aung San Suu Kyi, reversed years of progress toward democracy after five decades of military rule.

It has again made Myanmar the focus of international scrutiny as security forces have repeatedly fired into crowds of protesters.

As of Sunday, at least 459 people have been killed since the takeover, according to the Assistance Association for Political Prisoners, which has tallied deaths it was able to verify. The true toll is thought to be higher.

Australian Associated Press