Jan 11 (Reuters) - U.S. President Donald Trump said on Sunday he might veto legislation to extend federal health insurance subsidies, injecting fresh uncertainty into a debate that has pitted congressional Republicans against Democrats and threatened to raise premiums for millions of Americans.

Expiration of the health insurance tax breaks at the end of 2025 left millions of Americans facing significant price hikes for coverage.

The Republican-controlled U.S. House of Representatives on Thursday passed the Democratic-backed legislation that would restore the subsidies to the Affordable Care Act, with 17 Republicans joining Democrats.

The Senate, which is also controlled by Republicans, has already rejected a similar bill, but House passage could spur a compromise.

Americans have until January 15 to enroll in ACA coverage for this year, although the Trump administration could extend that deadline. (Reporting By Jarrett Renshaw and Julie Steenhuysen; Editing by Tom Hogue and Sergio Non)