SAN FRANCISCO — A split Ninth Circuit U.S. Court of Appeals today sided with the Obama administration in upholding an injunction that blocks the most controversial parts of Arizona’s tough new immigration laws.

A three-judge panel said the provisions, including those allowing warrantless arrests under certain circumstances and the criminalization of aliens’ unauthorized work, are likely preempted by federal law and therefore unconstitutional. The circuit reviewed Arizona District Judge Susan Bolton’s decision enjoining enforcement of the laws for an abuse of discretion.