North Korea says it has successfully tested an engine designed for an intercontinental ballistic missile.
The new type of engine would "guarantee" the ability to launch a nuclear strike on the US mainland, the KCNA news agency said.
The test was conducted at the country's long-range missile launch site near its west coast.
It is the latest in a series of tests and launches carried out by the isolated nation.
The United States issued a statement criticising North Korea's action.
"We call on North Korea to refrain from actions and rhetoric that further destabilize the region and focus instead on taking concrete steps toward fulfilling its commitments and international obligations," the state department said.
Leader Kim Jong-un supervised the test, state media report, during which "the engine spewed out huge flames with a deafening boom".
The country would now be able to "keep any cesspool of evils in the earth including the US mainland within our striking range," Mr Kim was quoted as saying.
Analysis: Steve Evans, BBC, Seoul
Step by step, North Korea is completing the tasks needed to have a nuclear-tipped missile capable of striking the US.
In recent weeks, it has said it has made the heat-resistant materials necessary. It broadcast pictures of Kim Jong-un looking on approvingly at what it claimed was a small nuclear warhead. Now it says it has the necessary rocket engine.
It is impossible to verify the claims, though on Tuesday the South Korean government accepted that North Korea had made crucial advances.
Next month, there is a grand political congress in Pyongyang to which Kim Jong-un is building up with fearsome rhetoric. He has threatened a fifth nuclear test. It would not be a surprise if that happened.
In March, North Korea said it had developed nuclear warheads small enough to fit on ballistic missiles.
However, experts cast doubt on the claims.
Last month also saw North Korea threaten "indiscriminate" nuclear strikes on the US and South Korea as they held big joint military drills, which the north sees as a rehearsal for an eventual invasion.
The US has also held talks with South Korea aimed at deploying a US missile defence system to the Korean peninsula, a move strongly opposed by North Korea, Russia and China.
Beijing says the Thaad anti-missile system compromises its security and would undermine its nuclear deterrent.
North Korea's rocket launches
- February 2016: Launch of rocket reportedly carrying satellite
- May 2015: North Korea announces it has successfully tested a submarine-launched missile for the first time, but scepticism is then poured on the claim
- Dec 2012: North Korea launches three-stage rocket, says it successfully put a satellite into orbit; US defence officials confirm object in orbit
- Apr 2012: Three-stage rocket explodes just after take-off, falls into sea
- Apr 2009: Three-stage rocket launched; North Korea says it was a success, US says it failed and fell into the sea
- Jul 2006: North Korea test-fires a long-range Taepodong-2 missile; US said it failed shortly after take-off