US President Donald Trump said Monday North Korea is a "real threat to the world," calling for the UN Security Council to get ready to impose tougher sanctions on the communist nation.

"The status quo in North Korea is also unacceptable," Trump said during a lunch meeting he hosted at the White House for UN ambassadors from countries that are members of the Security Council. "The council must be prepared to impose additional and stronger sanctions on North Korean nuclear and ballistic missile programs."

"This is a real threat to the world, whether we want to talk about it or not. North Korea is a big world problem and it's a problem we have to finally solve. People put blindfolds on for decades and now it's time to solve the problem," Trump said.

Trump also said the UN envoys will be "very busy people over the coming months and years."

North Korea and Syria were top issues during the lunch meeting, US Ambassador to the UN Nikki Haley said.

"It was an open dialogue very much of the members wanting to hear from the president what his plans are, what he is going to continue to do on Syria or not continue to do on North Korea. All those issues are certainly at the forefront," Haley told reporters at the White House briefing broom.

After the meeting, the UN diplomats attended a briefing by National Security Adviser H.R. McMaster "focusing on North Korea, also focusing on ISIS," Haley said.

Tensions with North Korea have escalated again in the run-up to Tuesday's anniversary of the founding of the North's military amid fears Pyongyang could mark the holiday with a nuclear test in what would be brazen defiance of the US and the international community.

Earlier in the day, Haley said the US could take military action against North Korea if the communist nation attacks a military base in the US or allies, or launches an intercontinental ballistic missile.

She also warned the North not to "give us a reason" to fight with the country.

"We're not going to do anything unless he gives us a reason to do something. So our goal is not to start a fight," Haley said on NBC's "Today" in response to a question if the US is seriously considering a preemptive strike against the North.

Asked what such a reason could be, the envoy said, "if you see him attack a military base, if you see some sort of intercontinental ballistic missile. Then obviously we're going to do that."

"But right now, we're saying, 'Don't test, don't use nuclear missiles, don't try and do any more actions' and I think he's understanding that. China is helping us really put that pressure on," Haley said.

Asked again if the US would take military retaliation if the North tests an ICBM or a nuclear device, Haley said, "I think then the president steps in and decides what's going to happen." (Yonhap)