President Donald Trump dismissed a Chinese diplomatic effort to rein in North Korea's weapons program as a failure on Thursday, while secretary of state Rex Tillerson said Beijing should do more to limit oil supplies to Pyongyang.
In a tweet, Trump delivered another insulting barb against North Korean leader Kim Jong Un, who he called "Little Rocket Man" and a "sick puppy" after North Korea test fired its most advanced missile to date on Wednesday.
"The Chinese envoy, who just returned from North Korea, seems to have had no impact on Little Rocket Man," Trump said on Twitter, a day after speaking with Chinese president Xi Jinping and reiterating his call for Beijing to use its leverage against North Korea.
However, Russian foreign minister Sergei Lavrov said Washington's approach was dangerously provocative.
Separately on Thursday, Mr Tillerson welcomed Chinese efforts on North Korea, but said Beijing could do more to limit its oil exports to the country.
"The Chinese are doing a lot. We do think they could do more with the oil. We're really asking them to please restrain more of the oil, not cut it off completely," Mr Tillerson said.China is North Korea's neighbour and its sole major trading partner.
While Mr Trump has been bellicose at times in rhetoric towards North Korea, Mr Tillerson has persistently held out hopes for a return to dialogue if North Korea shows it is willing to give up its nuclear weapons program.
However, Mr Tillerson may not remain in his job for long, with disagreements with the president over North Korea being one factor. On Thursday, a senior Trump administration official said the White House had developed a plan to replace Mr Tillerson with Mike Pompeo, the director of the Central Intelligence Agency.
In spite of Mr Trump's rhetoric and warnings that all options, including military ones, are on the table in dealing with North Korea, his administration has stressed it favours a diplomatic solution to the crisis, which stems from Pyongyang's pursuit of a nuclear-tipped missile capable of hitting the United States.
Tensions have flared anew after North Korea said it had successfully tested a new intercontinental ballistic missile on Wednesday in a "breakthrough" that put the US mainland within range of its nuclear weapons.
President Trump has pledged more sanctions in response to the latest test and at an emergency UN Security Council meeting late on Wednesday, the United States warned North Korea's leadership would be "utterly destroyed" if war were to break out.
Mr Lavrov pointed to joint US-South Korean military exercises planned for December and accused the United States of trying to provoke Kim into "flying off the handle" over his missile program to hand Washington a pretext to destroy his country.He also flatly rejected a US call to cut ties with Pyongyang over its nuclear and ballistic missile program, calling US policy towards North Korea deeply flawed.
"We have already said many times that sanctions pressure has exhausted itself," he said.
In a telephone call with Mr Trump on Thursday, South Korean President Moon Jae-in said the missile launched this week was North Korea's most advanced so far, but it was unclear whether Pyongyang had the technology to miniaturize a nuclear warhead and it still needed to prove other things, such as its re-entry technology.
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North Korea has tested dozens of ballistic missiles under Mr Kim's leadership and conducted its sixth and largest nuclear bomb test in September, claiming its weapons programme is a necessary defence against US invasion. The United States has 28,500 troops in South Korea as a legacy of the 1950-53 Korean War, but denies any such intention.
Previous US administrations have failed to stop North Korea from developing nuclear weapons and a sophisticated missile programme. Mr Trump, who has previously said the United States would "totally destroy" North Korea if necessary to protect itself and its allies from the nuclear threat, has also struggled to contain Pyongyang since taking office in January.
China has so far refrained from complying with American requests to cut off its oil supply to North Korea. Moscow also sells oil products to North Korea and thousands of North Koreans work in Russia, sending money back to the authorities in Pyongyang.