(Reuters) - A U.S. judge on Wednesday blocked key parts of Arizona's tough new immigration law, granting the Obama administration's request for an injunction on grounds that immigration matters are the federal government's responsibility.

U.S. District Judge Susan Bolton agreed to an injunction on provisions including one that required a police officer to determine the immigration status of a person detained or arrested if the officer believes the person is not in the country legally.

The Republican-controlled Arizona Legislature passed the law in April to try to stem the flow of illegal immigrants over the state's border with Mexico and cut down on drug trafficking and crime -- setting it on a collision course with the federal government.