A top aide to U.S. President Donald Trump expressed concern Thursday that North Korea's nuclear weapons development program and a possible future nuclear threat from Iran may spark the proliferation of weapons of mass destruction around the world.

"If eventually other countries...become nuclear powers, there'll be a real impetus for a lot of countries to develop or buy nuclear weapons," Trump's chief of staff John Kelly told reporters at the White House.

On North Korea, Kelly said Americans "should be concerned about a state that has developed a pretty good ICBM capability and is developing a pretty good nuclear re-entry vehicle. I would believe, I think I speak for the administration, that that state simply cannot have the ability to reach the homeland."

Giving his first press conference since assuming his post in July, Kelly noted that there is "great concern about a lot of Americans that live in Guam."

"Right now we think the threat is manageable but over time, if it grows beyond where it is today, well, let's hope that diplomacy works," he said.

Tensions have been running high in Northeast Asia since North Korea tested a series of ballistic missiles, including two intercontinental ballistic missiles in July, and the country's sixth and most powerful nuclear test in September.