Nayna Gupta of American Immigration Council says US authorities go after non-citizens to rack up their arrest numbers.
An Iranian spokesperson called the move a sign of a 'supremacist and racist mentality' dominating US policy.
Defence Minister Phumtham Wechayachai says Thailand reinforces military presence in response to Cambodia move.
More than 40 people arrested by immigration officials in 'oppressive and vile paramilitary operation', advocates say.