U.S. official says Secretary of State Condoleezza Rice has put off plans to visit Canada because of that country's decision not to take part in a planned U.S. missile defense system.

Officials have not announced a new date for the trip, which was reportedly planned for mid-April.

The delay could signal a new strain in U.S.-Canadian relations, already cooled by Ottawa's opposition to the war in Iraq.

State Department spokesman Richard Boucher says U.S. officials still look forward to seeing the Canadians soon, and are searching for a date to schedule a meeting.

The planned U.S. missile defense system is designed to protect the U.S. mainland by shooting down an incoming ballistic missile. The system has failed two major tests since December.

Some information for this report provided by AP and Reuters.