WASHINGTON • The Trump administration is using the vast and nearly opaque immigration bureaucracy of the United States to constrict the flow of foreign workers into the country by throwing up new roadblocks to limit legal arrivals.

The government is denying more work visas, asking applicants to provide additional information and delaying approvals more frequently than just a year earlier. Hospitals, hotels, technology companies and other businesses say they are now struggling to fill jobs with the foreign workers they need.

With foreign hires missing, the employees who remain are being forced to pick up the slack.

Seasonal industries such as hotels and landscaping are having to turn down customers or provide fewer services. Corporate executives worry about the long-term effect of losing talented engineers and programmers to countries like Canada that are laying out the welcome mat for skilled foreigners.

In April last year, President Donald Trump signed a "Buy American and Hire American" executive order, directing government officials to "rigorously enforce" immigration laws. The order did not get the kind of attention that followed the administration's decision to separate families at the Mexican border this summer.

A few months later, the President endorsed legislation that would cut legal immigration by half. The Bill was introduced by Republican senators Tom Cotton and David Perdue. But Republican leaders in Congress have not advanced it.

Some lawmakers say Mr Trump is using administrative means to reshape immigration policy because those changes have stalled on Capitol Hill.

"If they want to have a proposal on immigration, they should send it to Congress," said Democratic Congressman Ro Khanna. "The administration should engage in that conversation. To unilaterally and without any accountability change what Congress has authorised is not democratic."

A recent analysis of government data by the National Foundation for American Policy, a nonpartisan research group, found that the denial rate for H-1B visa petitions for skilled foreign workers had increased by 41 per cent in the last three months of the 2017 fiscal year, compared with the third quarter.

Government requests for additional information for applications doubled in the fourth quarter, a few months after Mr Trump issued his order.

Experts say a sustained reduction in immigration could dampen growth over time as more baby boomers retire, leaving big gaps in the job market.

That goes for high-skilled immigrants and low-skilled workers, said Ms Francine Blau, an economist at Cornell. The latter will be vital in fields like eldercare and childcare, as well as construction and cleaning.

"A lot of our labour force growth comes from immigrants and their children," Ms Blau said. "Without them, we'd suffer the problems associated with countries with an ageing population, like Japan."

NYTIMES