Donald Trump and his South Korean counterpart Moon Jae-in have agreed to "maximise pressure" on Pyongyang after the US President approved a weapons deal with Seoul worth billions of dollars.
Mr Trump pledged to strengthen joint military efforts and approved to lift restrictions on South Korea's missile payload capabilities.
The two leaders spoke on the phone, and said they would "use all means at their disposal" to up pressure on North Korea.
The news comes after Mr Trump has lambasted China for "failing" to help in the effort against North Korea and the US ambassador for the UN, Nikki Haley, said North Korea's leader is "begging for war".
Ms Haley said at an emergency meeting of the UN Security Council that the US would look at countries doing business with the regime and circulate a resolution this week and try and get it approved before 11 September.
"Enough is enough," she said.
"War is never something the United States wants. We don't want it now. But our country's patience is not unlimited.
“The United States will look at every country that does business with North Korea as a country, that is giving aid to their reckless and dangerous nuclear intentions.”
North Korea said it tested an advanced hydrogen bomb for a long-range missile on Sunday, prompting consternation from world leaders.
The UN Security Council meeting came just six days after it condemned Pyongyang's "outrageous" launch of a ballistic missile over Japan.
The US promised a "massive" military response if it or its allies were threatened.
Ideas how to approach the regime differ. Russia and China have suggested that North Korea suspend its nuclear and missile development, but that also the US and South Korea stop their joint military exercises.
Pyongyang has blasted the joint exercises, viewing them as a rehearsal for invasion, while the US and South Korea say they are acting defensively.
Mr Trump and German Chancellor Angela Merkel urged the United Nations to quickly agree on tougher sanctions against Pyongyang, with German authorities calling the missle test a "new and unacceptable escalation" by the North Korean regime.
Ms Merkel told Mr Trump she would push for tougher sanctions from the European Union.
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