U.S. Secretary of State Rex Tillerson urged China to cut oil exports to North Korea to force Pyongyang to rein in its nuclear weapons program during his visit to London on Thursday.
Tillerson's trip to Britain comes days after the United Nations Security Council imposed new sanctions on North Korea. He said he had hoped for stronger measures from the U.N., and urged Beijing to use its leverage.
"I am hopeful that China, as a great country, as a world power, will decide on their own and will take it upon themselves to use that very powerful tool of oil supply to persuade North Korea to reconsider its current path toward weapons development, reconsider its approach to dialogue and negotiations in the future," he told reporters following meetings with his British and French counterparts.
Tillerson also had strong words for Iran, which he said was "clearly in default" of its expectations over the nuclear agreement. Britain supports the deal, but the United States accuses Tehran of breaching the terms.
"We must take into account the totality of Iranian threats — not just Iran's nuclear capabilities, that is one piece of our posture toward Iran," Tillerson told reporters.
Alongside his British hosts, Tillerson attended a summit on Libya with the country's U.N. special representative, and delegations from France, the United Arab Emirates, Italy and Egypt.
He said the United Nations has Washington's full backing in seeking a political settlement.
"What we don't want to see happen is Libya becoming a place to birth additional terrorist organizations, or provide opportunities for ISIS to re-emerge in a different part of the world. We are all committed to helping the Libyans find a Libyan solution that will lead to their future," Tillerson said. ISIS is an acronym for Islamic State.
WATCH: In London, U.S. Secretary of State Chides Iran, Urges China to Block North Korean Oil
British Foreign Secretary Boris Johnson expressed hope that elections could be held in Libya within a year.
"It's very important, however, that you don't do it too fast, and that you get the political groundwork done first. There has to be a constitution, there has to be an accepted basis for those elections to take place," Johnson said.
There are growing fears, though, that the Islamic State terror group is making a comeback after being ousted in December from its stronghold city of Sirte, Libya.
Militias are exploiting the standoff between the internationally recognized Tripoli-based government and its rival administration in the east, said Riccardo Fabiani of the Eurasia Group.
"Those militiamen and jihadis that were part of the group were going to reform at some point somewhere, and to mount new attacks. So this is not something that is going to disappear overnight and will continue to be there as long as there is insecurity and instability in Libya," Fabiani said.
Tillerson also was questioned on U.S. support for the Myanmar government in the wake of the attacks on ethnic Rohingya Muslims. He said the military should take the blame.
"This violence must stop. This persecution must stop. It's been characterized by many as ethnic cleansing — that must stop," Tillerson said. "And we need to support [Myanmar State Counselor] Aung San Suu Kyi and her leadership. But also be very clear and unequivocal to the military power-sharing government that this is unacceptable."
In closing remarks, Tillerson said Britain faced challenges over Brexit, but reiterated that the United States would be a steadfast ally.
Analysts said Britain is keen to underline its ambitions of remaining a global player after its exit from the European Union. London sees its relationship with the United States as key to that goal.