US Secretary of State Rex Tillerson has said it is "very difficult" to get China to agree to an oil embargo on North Korea, but he is hopeful that the "great country" will decide to use its "powerful tool" of oil supplies to persuade Pyongyang to reconsider its development of nuclear weapons.

Tillerson's remarks came on Thursday at a joint news conference with UK's Foreign Minister Boris Johnson in London where he met British Prime Minister Theresa May and French President Emmanuel Macron.

He said the United States has sought an embargo on oil imports to North Korea at the UN Security Council in response to North Korea's most powerful nuclear test to date.

But the UN agreed last week to weaker measures against Pyongyang - although the UN is banning ban textile exports, an important source of revenue for North Korea.

"It's clear that with respect to oil, and a complete embargo on oil from the UN Security Council, that's going to be very difficult," he said.

"In effect, that is directed at China alone because China supplies essentially all of North Korea's oil.

"I am hopeful that China, as a great country, a world power, will decide on their own, will take it upon themselves to use that very powerful tool of oil supply to persuade North Korea to reconsider its current path towards weapons development, reconsider its approach to dialogue and negotiations in the future," he said.

Al Jazeera's senior political analyst Marwan Bishara, speaking from London, said China and Russia believe that stripping North Korea of everything will push Pyongyang to war.

"All in all the use of threatening words towards China is of course counterproductive. And at the end of the day everyone knows that the solution is diplomatic and you cannot have a diplomatic solution without China."