奥巴马敦促加强导弹防御系统以应对朝鲜威胁

U.S. President Barack Obama is calling for a strengthened missile defense system in order to counter the threat of North Korea, which defied U.N. sanctions by conducting its third nuclear test.

Speaking Tuesday at the annual State of the Union address, President Obama vowed to take "firm action" in response to the test.

Mr. Obama's speech came just hours after the U.N. Security Council unanimously condemned the underground nuclear test, calling it a "grave violation" of previous international sanctions against North Korea's weapons program.