Mexico President Andrés Manuel López Obrador said his government has rejected Democratic lawmakers' plan to allow US inspectors to check Mexican factories for labour law compliance.
Mexico wants the US Congress to approve a new North American trade accord before US elections next year complicate the picture.
Democrats are concerned Mexico's unrepresentative unions have led to low wages that drain manufacturing jobs from the US.
López Obrador noted Mexico had passed reforms requiring free and fair union elections in its factories and approved a budget for monitoring.
He said some Democrats continue raising concerns about implementation and compliance and requested inspectors.
"We don't accept that," he said.
Mexico countered with an idea of panels that would include representatives of both countries and a third nation to rule on controversies.
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