WASHINGTON • The US administration has begun to deliver on President Donald Trump's campaign promise to crack down on a work visa programme that channels thousands of skilled overseas workers to companies across the technology industry.

The moves include new measures announced on Monday to detect "fraud and abuse" in the H-1B visa programme, and a warning by the Justice Department to firms not to discriminate against US workers.

Guidelines released last week will require a stricter definition of the skill levels for computer programmers to qualify for the temporary visa programme for jobs that cannot be filled by Americans.

These moves come as the United States opens up its annual allotment for the H-1B visa programme - a total of 85,000 slots which are quickly snapped up each year, notably by tech firms.

As per the new guidelines, additional information is required for computer programmers applying for H-1B visas to prove the jobs are complicated and require more advanced knowledge and experience.

It is effective immediately, so it will change how companies apply for the visas in an annual lottery process that began on Monday.

"Each of these steps are small steps by themselves," said Mr R. Chandrashekhar, president of the trade group Nasscom, which represents many Indian tech firms.

"What we are waiting to see is how they will tighten the process."

The changes do not explicitly prohibit applications for a specific type of job.

Instead, they bring more scrutiny to those for computer programmers doing the simplest jobs.

Computer programmers made up about 12 per cent of all H-1B applications certified by the Department of Labour in 2015.

Of those, 41 per cent were for positions at the lowest wage level, defined as jobs requiring people to perform routine tasks that require them to exercise little judgment on their own.

"You're going to have to beef up your argument for why you need this person," said Howard University's Associate Professor Ron Hira, who has done extensive research on the H-1B programme.

Silicon Valley's biggest tech firms, many of which were founded or run by immigrants, depend on H-1Bs and say efforts to thwart immigration threaten innovation, recruitment and start-up formation.

But there is also broad recognition that reform is needed, given several high-profile examples where American employees have been replaced by lower-paid foreign workers through the programme.

Tech company stocks took a hit after the announcement on Monday, with shares of Cognizant Technology Solutions, Infosys, Wipro and Accenture each slipping more than 1 per cent.

BLOOMBERG, AGENCE FRANCE-PRESSE