The U.S. has sent fighter jets to join military exercises in South Korea as tensions rise with North Korea.
Over the weekend, state news reported North Korea said it had entered a "state of war" with South Korea.
South Korea's president warns they'll strike back quickly to any attack.
North Korea has made threats against both South Korea and the U.S. since new U.N. sanctions were imposed.
MORE: South Korea warns North of 'strong response' to any attack
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