The United States has positioned warships closer to the Korean peninsula as North Korea denounced President Trump's air strikes in Syria as an "unforgivable act of aggression".
The USS Carl Vinson aircraft carrier and other warships have been sent to the Western Pacific, detouring from a planned route to Australia as a "prudent measure".
"The number one threat in the region continues to be North Korea, due to its reckless, irresponsible and destabilising programme of missile tests and pursuit of a nuclear weapons capability," a spokesperson for US Pacific Command told the BBC.
North Korea's foreign ministry responded to US action in Syria on Saturday. According to the country;'s official news agency, it said: "The US missile attack against Syria is a clear and unforgivable act of aggression against a sovereign state and we strongly condemn this."
Trump last week warned that he would not rule out taking action against North Korea, ahead of his a meeting with China's Xi Jinping this weekend.
North Korea tested ballistic missile capabilities in February, its first since Trump's election. The secretive communist country conducted more than 20 tests last year and two nuclear tests.