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