Two U.S. strategic bombers conducted drills over South Korea on Thursday, the U.S. Air Force said, raising tensions with North Korea just days before President Donald Trump visits the region seeking to shut down Pyongyang's nuclear programme.



News of the drills was first reported by North Korean state news agency KCNA on Friday, which said the exercises involving South Korean and Japanese fighter jets were a "surprise nuclear strike drill".



"The reality clearly shows that the gangster-like U.S. imperialists are the very one who is aggravating the situation of the Korean peninsula and seeking to ignite a nuclear war," KCNA said.



Trump arrives in Asia on Sunday, beginning his first trip to the region as president in Japan before heading to South Korea and China, then Vietnam and the Philippines.



A series of missile tests by North Korea and its sixth and largest nuclear test have fueled the most critical international challenge of Trump's presidency.



The goal of Trump's visit will be to increase international support for efforts to deprive North Korea of resources as leverage to coerce it to give up nuclear weapons, U.S. officials said.



"The president recognizes that we're running out of time (to deal with North Korea) and will ask all nations to do more," White House national security adviser H.R. McMaster told reporters at a briefing in Washington.



McMaster said Trump, who has approved a variety of sanctions against North Korea while pressing China to do more, is at the beginning of his drive for Pyongyang to give up nuclear weapons. Trump has warned he would "totally destroy" North Korea if it threatened the United States.



"I think we have to be a little patient here for at least a few months to see what more we and others can do, including China," said McMaster. "I don't think we need to reassess our strategy now. I think we have to give it a couple of months, a few months, and then see what adjustments we might need to make."



China's influential state-run tabloid the Global Times said in an editorial on Friday that pressing China will not help solve the Korean peninsula nuclear crisis.



"The complexity of the nuclear crisis means that all sides may have to make some concessions to reach a peaceful solution. China is playing the most difficult role in the process and is the real hope of peacefully addressing the crisis. Neither side should press China in an extreme way," it said.