North Korea must "earn its way back" to the negotiating table, US Secretary of State Rex Tillerson said Friday, backing away from an earlier offer of unconditional talks to end the stand-off with Pyongyang.

Tillerson told the UN Security Council that a "sustained cessation of North Korea's threatening behavior must occur before talks can begin," apparently after coming under pressure from President Donald Trump's White House to toughen his stance.

Earlier this week at a policy forum in Washington, Tillerson had underlined that the United States was seeking a diplomatic opening for negotiations with North Korea on ending its nuclear program and, for the first time, offered "talks with no preconditions."

The White House had responded to Tillerson's remarks by insisting there had been no change in US policy, triggering a new round of reports of a rift between Trump and his chief diplomat.

At the United Nations, Tillerson took a tough line.

"North Korea must earn its way back to the table," he said. "The pressure campaign must and will continue until denuclearization is achieved."

"We will, in the meantime, keep our channels of communication open."

Tillerson repeated that "we do not seek, nor do we want, war with North Korea."

"The United States will use all necessary measures to defend itself against North Korean aggression, but our hope remains that diplomacy will produce a resolution," he said.

According to a prepared version of his remarks, Tillerson had planned to repeat his offer of talks without preconditions, but the line was dropped from his speech.

Asked about the change, Tillerson told reporters afterwards that the United States would not make any concessions of its own to secure talks, but that lines of communication would remain open if North Korea wanted to come forward.

The North Koreans "know where the door is. They know where to walk through that door when they want to talk," he said.

North Korea's UN ambassador Ja Song-Nam made a rare appearance at the council meeting chaired by Japan, which announced new targeted sanctions on Chinese entities, among others.

Ja took a swipe at the United States which he said was "terrified by the incredible might of our republic" and reaffirmed Pyongyang's view that nuclear weapons were needed for self-defense.

North Korea is "a responsible nuclear power and a peace-loving state" that will strive to safeguard world peace, he said, making no mention of the US call for a pause in missile and nuclear tests.

Singling out China, Pyongyang's sole ally, and Russia, Tillerson urged Beijing and Moscow to go beyond the current tough UN sanctions resolutions and impose unilateral measures.

"Continuing to allow North Korean laborers to toil in slave-like conditions inside Russia in exchange for wages used to fund nuclear weapons programs calls into question Russia's dedication as a partner for peace," he said.

Tillerson also dismissed concerns that UN sanctions are having an impact on North Korea's humanitarian crisis, saying Pyongyang "hypocritically spends billions" on military programs "while its own people suffer great poverty."

The United States has called on China to cut off oil supplies to North Korea, a move that would deal a crippling blow to its economy.

In Washington, Trump criticized Russia for doing too little to pressure Pyongyang.

 "China is helping. Russia is not helping. We would like to have Russia's help," said Trump, who discussed the crisis by phone with President Vladimir Putin on Thursday.

Russia's Ambassador Vassily Nebenzia shot back, saying North Korean workers were in Russia as part of a bilateral agreement "which guarantees their rights"

-- and not subjected to slave-like conditions.

 Addressing Tillerson directly, the Russian ambassador said: "We very much hope that the US will be able to help resolve the crisis on the Korean peninsula."

China's envoy expressed concern about a hardening of positions, saying the current crisis "hardly makes one optimistic".

"The hope for peace is not totally obliterated," said Chinese Deputy Ambassador Wu Haitao. There is "a possibility for negotiation and the option of use of force is unacceptable."

Pyongyang has carried out its sixth nuclear test and conducted a series of missile launches including its first tests of two intercontinental ballistic missiles (ICBM) this year.

Addressing the council, UN Secretary-General Antonio Guterres urged diplomatic engagement to avoid a "level of danger that would be unpredictable in its trajectory and catastrophic in its consequences."

The UN's political affairs chief, Jeffrey Feltman, returned at the weekend from talks in Pyongyang -- the first visit to the North by a high-ranking UN official since 2011.

While Feltman received no firm commitment on talks from North Korea, he said the visit was "just the beginning" of his push for "talks about talks."

Since late 2016, the Security Council has slapped three rounds of sanctions aimed at choking off revenue to Pyongyang's weapons program, which the United Nations has described as the most dangerous security issue in the world today.

China and Russia argue that sanctions alone will not compel North Korean leader Kim Jong-Un to change course and want to step up diplomatic efforts to achieve a solution. (AFP)