A U.S. labor agency has concluded that ride-hailing company Uber Technologies Inc's drivers are independent contractors and not its employees, which could prevent them from joining unions.

The National Labor Relations Board's general counsel, in a memo released on Tuesday, said Uber drivers set their hours, own their cars and are free to work for the company's competitors, so they cannot be considered employees under federal labor law.

San Francisco-based Uber in a statement said it is 'focused on improving the quality and security of independent work, while preserving the flexibility drivers and couriers tell us they value.'

Uber shares were up 6.4 percent at $39.46 in late trading on the New York Stock Exchange.

The memo dated April 16 came in an NLRB case against Uber that has yet to reach the five-member board, which is independent of the general counsel.

Under the National Labor Relations Act, independent contractors cannot join unions and do not have legal protection when they complain about working conditions.

In January, President Donald Trump's appointees to the NLRB adopted a new test making it more difficult for workers to prove they are a company's employees.

Uber, its top rival Lyft Inc, and many other 'gig economy' companies have faced scores of lawsuits accusing them of misclassifying workers as independent contractors under federal and state wage laws.

Employees are significantly more costly because they are entitled to the minimum wage, overtime pay and reimbursements for work-related expenses under those laws.

Uber, in a filing with the U.S. Securities and Exchange Commission last week, said it would pay up to $170 million to settle tens of thousands of arbitration cases with drivers who claim they were misclassified. Uber denied any wrongdoing, but said settling the cases was preferable to drawn-out litigation.

The company has agreed to pay an additional $20 million to end long-running lawsuits by thousands of drivers in California and Massachusetts.

The U.S. Department of Labor in a memo released last month said an unidentified 'gig economy' company's workers were not its employees under federal wage law because it did not control their work.

The company, which appeared from the memo to provide house-cleaning services, had a similar relationship with its workers as Uber does with drivers. The memo signaled a shift from the Obama administration, which maintained that most workers should be considered companies' employees.