UNITED NATIONS, Sept 3 (Reuters) - U.N. Secretary-General Ban Ki-moon said on Tuesday that the use of force is only legal when it is in self-defense or with U.N. Security Council authorization, remarks that appear to question the legality of U.S. plans to strike Syria without U.N. backing.

He said that if U.N. inspectors confirm the use of chemical weapons in Syria, the Security Council, which has long been deadlocked on the 2-1/2-year Syrian civil war, should overcome its differences and take action.

"If confirmed, any use of chemical weapons by anyone under any circumstances will be a serious violation of international law and outrageous war crime," he told reporters. "Any perpetrators must be brought to justice. There should be no impunity." (Reporting by Louis Charbonneau; Editing by Sandra Maler)

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