President Donald Trump's crackdown on the controversial H-1B visa is wreaking havoc on U.S. employers, says a group whose members include many of Silicon Valley's largest technology firms.

In a letter to U.S. Citizenship and Immigration and the Department of Homeland Security, industry group Compete America said Citizenship and Immigration's approach to deciding who gets an H-1B was "leaving employers with a disruptive lack of clarity about the agency's practices, procedures, and policies."

Compete America said its members were reporting a "dramatic increase" over the past 18 months in the number of H-1B applications denied or held up by demands for more information, and a "sharp increase" in notices of intent to deny or revoke H-1B visas.

The H-1B, intended for jobs requiring specialized knowledge and a bachelor's degree or higher, has become a flash point in America's immigration debate, with tech companies pushing for an expansion of the annual 85,000 cap on new visas, and critics charging that U.S. firms use it to supplant American workers with cheaper foreign labor.

Compete America, which represents companies including Google, Facebook, Cisco, Hewlett-Packard Enterprise, Oracle, Salesforce, Amazon, Microsoft, IBM and Walmart, plus outsourcing and consulting firms Accenture and Deloitte, suggested in its Nov. 1 letter that federal authorities were denying and obstructing H-1B applications for improper reasons. Citizenship and Immigration has taken aim at applications for jobs with entry-level wages, and also at applications for jobs it says don't match applicants' degree types, Compete America said.

The group also pushed back against what it said were denials by Citizenship and Immigration based on the idea that H-1Bs should be used only for jobs requiring a bachelor's degree or higher. Such a degree should not always be necessary for granting an H-1B, the group said.

Compete America wants USCIS and the Department of Homeland Security to review current H-1B adjudications and practices, and provide any needed clarification internally or with "the regulated community."

The reported effects of the Trump administration's clampdown on the H-1B come as the administration moves forward with its plans to change the way the H-1B lottery is run in order to favor workers with higher education levels, and to strip work authorization from spouses of H-1B holders on track for green cards.

It's unclear when the lottery change may occur, but the federal government has said it will announce the work ban this month. A comment period on the proposed employment prohibition is expected to follow the announcement before it would go into effect, but federal rules can be imposed without a comment period.