The founder of the Free Burma Rangers said 2,009 people had been forced to return to the Ee Thu Hta displacement camp on the Myanmar side of the border.
Thai authorities have forced thousands of refugees fleeing air attacks by the army in neighbouring Myanmar to return to southeastern Karen state, two activist groups said.
Thousands fled over the weekend after fighter jets attacked villages near the Thai border held by an ethnic armed group that had attacked a military post in the wake of a February 1 coup by Myanmar's army.
David Eubank, founder of the Free Burma Rangers, said 2,009 people had been forced to return to the Ee Thu Hta displacement camp on the Myanmar side of the border at 6:15 pm.
They had been living there since they were displaced from their homes in earlier attacks.
"There's still fighter jets over the area," Mark Farmaner, head of Burma Campaign UK said.
"Thailand’s heartless and illegal act must stop now," Sunai Phasuk, senior researcher on Thailand for Human Rights Watch, wrote on Twitter.
Urgent! #Thai military pushed back over 2,000 asylum seekers, who crossed Salween River from #Myanmar yesterday to escape #Tatmadaw’s brutal offensives in Karen State. #Thailand’s heartless and illegal act must stop now. #WhatsHapppeningInMyanmar pic.twitter.com/GdtuMEkk9P— Sunai (@sunaibkk) March 29, 2021
A Thai provincial official from the Mae Hong Son district who declined to be named said the group was not pushed back.
"They are in Thai territory by the Salween River but they haven't come further. It's under army management," the official said.
Thai Prime Minister Prayuth Chan-ocha said earlier on Monday the government was prepared to accept refugees and rebuffed claims Thailand was supporting the Myanmar junta, telling reporters, "There is probably no one to support the use of violence against the people."
At least 459 killed
Myanmar's security forces have killed at least 459 people since the coup as it seeks to crush mass protests, according to the Assistance Association for Political Prisoners.
Hundreds of people including politicians from the former civilian government have fled central areas and taken shelter in territory held by ethnic armed groups.
Myanmar's military has for decades justified its grip on power by saying it is the only institution capable of preserving national unity. It seized power saying that November elections won by Nobel laureate Aung San Suu Kyi's party were fraudulent, an assertion dismissed by the election commission.