US President Donald Trump on Tuesday signed an executive order that could tighten the H-1B immigrant visa program that is popular with skilled workers from India and China.

He signed the order during a visit to the headquarters of Snap-On Inc, a tool manufacturer in Kenosha, Wisconsin, saying that H-1B visas should "never be used to replace Americans", adding that "we are about to take bold new steps to follow through on my pledge to buy American and hire American".

"Right now, widespread abuse in our immigration system is allowing American workers of all backgrounds to be replaced by workers brought in from other countries," he said.