Under the 2012 agreement with the US, South Korea is now allowed to develop missiles with a range of up to 800km (497 miles) and a payload of 500kg, according to the Nuclear Threat Initiative (NTI).
"Today's news makes clear that much more than these sanctions is required to protect US interests, said Jay Lefkowitz, a former USA special envoy on human rights in North Korea".
The Washington Post first reported that the nation had such capability, based on a Defense Intelligence Agency assessment read to the newspaper it verified with two us officials.
Washington has repeatedly pushed for the denuclearization and disarmament of North Korea, while it conducts joint drills with its allies in the region, including South Korea and Japan.
In response to queries from Yonhap, both the Pentagon and the State Department declined to comment, saying the assessment falls under the realm of intelligence. But he said the USA should assume the worst.
There are significant differences between Republicans and Democrats when it comes to the use of force against North Korea.
The UN Security Council agreed on Saturday to put in place a new set of economic sanctions targeting key sectors including coal and iron as well as seafood exports and North Korean laborers overseas, in hopes of hitting Pyongyang's billions of dollars of export revenue.
Navy personnel sit in front of a submarine-launched "Pukguksong" ballistic missile as it is paraded through Pyongyang, North Korea.
The sanctions are the toughest of the seven Security Council resolutions adopted since 2006 aimed at curbing North Korea's nuclear militarisation.
In June for instance, Trump caused mystification and a temporary blip in Asian regional tensions, when he tweeted: "While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out". The missiles, according to Pyongyang and many experts, have the range to strike the U.S. mainland.
The Post said a separate intelligence assessment concluded the North has built up to 60 nuclear warheads, a number far larger than independent experts' estimates.
On North Korean leader Kim Jong Un, 91 percent of respondents view him unfavorably and 6 percent view him favorably, the poll shows.
