White House Chief of Staff John Kelly said Americans should be concerned about North Korea’s nuclear capability.
According to Mr Kelly, Pyongyang “is developing a pretty good nuclear re-entry vehicle”.
He said: “The American people 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.
For a missile to hit its target successfully it would have to re-enter the Earth’s atmosphere without breaking up.
The Chief of Staff’s comments suggest that North Korea is close to achieving its goal.
Mr Kelly added: “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.”
It comes as South Korean and US military officers have spoken on the phone to discuss cooperation.
Chairman of the Joint Chiefs of Staff General Jeong Kyeong-doo and his US counterpart, Joseph Dunford, spoke by phone.
Dunford stressed the US-South Korea alliance remains firm and their military relations will advance further, according to Jeong’s office.
The phone call marks the third of its kind since Jeong took office in August.
It comes following months of tensions with North Korea following tests of intercontinental ballistic missiles which were first tested in July.
Last month, the hermit kingdom tested a missile capable of reaching continental US.
President Trump met with top defence officials on Tuesday to discuss his options.
In a statement, the White House said: “This morning President Donald Trump met with members of his national security team to receive a briefing from Secretary of Defence James Mattis and Chairman of the Joint Chiefs of Staff General Joseph Dunford.
“The briefing and discussion focused on a range of options to respond to any form of North Korean aggression or, if necessary, to prevent North Korea from threatening the United States and its allies with nuclear weapons.”
After the meeting, the US flew two supersonic bombers over the Korean peninsula.
Donald Trump’s North Korea meeting was held in the situation room in the White House where US Presidents traditionally hold “war councils” at times of conflict.
The most senior US general recently told the American army to “stand ready” for war with North Korea.