The White House has repeatedly called on North Korea to abandon its nuclear and missile programs.
The North's Foreign Ministry "condemns and outrightly rejects the sanctions racket put forth by the United States and the U.N. Security Council to prevent the strengthening of our nuclear deterrence", a ministry spokesman said.
The UN Security Council on Friday expanded targeted sanctions against North Korea after its repeated missile tests, adopting the first such resolution agreed by the United States and Pyongyang's only major ally China since President Donald Trump took office.
Among the four entities listed in the resolution are Kangbong Trading Corp., Korea Kumsan Trading Corp., Koryo Bank and the Strategic Rocket Force of the Korean People's Army.
These partners include China, with Mattis saying that the Trump administration is encouraged by its " renewed commitment to work with the worldwide community toward de-nuclearization, " adding that he believed China will eventually see North Korea as a strategic liability and not an asset given its propensity for inciting disharmony.
While he admitted that rivalry between China and the United States - the world's two largest economies - was bound to occur, he also said that did not mean conflict was inevitable. Ambassadors of countries such as France and Italy supported the resolution but stressed that sanctions are only a means to an end, not a goal realised.
Once considered a strong ally, Russian Federation has backed fresh sanctions against North Korea.
Among those added to the sanctions blacklist was Cho Il-U, believed to be the head of foreign espionage for Kim Jong-Un's regime.
The Trump administration has pledged to work with China on North Korea, but Trump has said the USA would act unilaterally if China did not step in.
In his speech, Mattis acknowledged the hard diplomacy involved in getting China to act against North Korea, a longtime trade partner that shares a border.
This is a "reaffirmation of the US' longstanding interest in and commitment to the region", said Singapore's Defence Ministry.
The security summit is taking place amid anxieties over what appears to be a U.S. retreat from the region after Mr Trump pulled out of the 12-nation Trans-Pacific Partnership trade deal and denounced his predecessor's pivot to the region.
"The United States is fully committed to defending ourselves and our allies against North Korean aggression", Haley said.
"Once we've exhausted all possible alternatives, the Americans will do the right thing".
Since Moon's May 10 inauguration, North Korea has test-fired three ballistic missiles in an apparent show of its resolve to expand its weapons arsenal to cope with what it calls US hostility.
Experts seem to think the recent ramp-up in United States military presence in the region seems only to be escalating the situation.
Defence chief James Mattis says Pyongyang's intensified pursuit of nuclear weapons threatens all nations.
The latest resolution condemned "in the strongest terms" North Korea's missile and nuclear weapons activities and demanded that Pyongyang abandon all of its military programs.
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