The United States has censured North Korea over ‘destabilizing’ the region with what Washington has described as Pyongyang’s Intercontinental Ballistic Missile (ICBM) program.
"We have seen the reports of North Korea's claims to have developed new engine technology for its ICBMs," said a statement by State Department deputy spokesman Mark Toner.
"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 statement follows an earlier announcement by North Korea on Saturday in which Pyongyang said it successfully tested an engine designed for an ICBM that will give it the ability to stage nuclear attacks on the United States.
"The great success ... provided a firm guarantee for mounting another form of nuclear attack upon the US imperialists and other hostile forces," North Korea’s official news agency, KCNA, quoted leader Kim Jong-un as saying.
The alleged test followed Pyongyang’s firing of a missile into the sea off its eastern coast on April 1.
The tests were conducted in defiance of sanctions by the UN Security Council that ban North Korea from carrying out activities related to ballistic missile.
Following the North's fourth nuclear test in January and an alleged satellite launch the next month, the US and China formed a front in the UN to slap new sanctions on Pyongyang.
On Friday, Defense Secretary Ashton Carter said the United States will go ahead with a proposed deployment of a US missile system in South Korea despite Chinese complaints.
Carter said the plan was necessary to protect both American forces on the Korean Peninsula and South Korea from the North.