Donald Trump has suggested China may move to "end this nonsense once and for all"
North Korea claims to have successfully tested its first intercontinental ballistic missile - contradicting US and South Korean officials who earlier said it was an intermediate-range missile.
The latest in a series of missile launches in recent months appears to be the secretive state's longest-range ballistic missile to date.
It is believed to have taken place from Pyongyang's North Phyongan province and flew 930km in about 40 minutes before landing in the Sea of Japan.
South Korean President Moon Jae-in said: "We assume it a medium long-range ballistic missile. But we still plan to devise necessary measures assuming it may have been an ICBM (Intercontinental Ballistic Missile).
"If it is ICBM, we will need to come up with the corresponding measures."
He added Seoul would continue to "resolutely deal with North Korean provocations in close co-operation with the international community while maintaining a strong defence with the (South) Korea-US joint forces, based on the strong Korea-US alliance".
It is the latest in a series of test firings as Pyongyang attempts to build a missile that could reach the United States.
US President Donald Trump criticised the launch and North Korean leader Kim Jong-un:
North Korea has just launched another missile. Does this guy have anything better to do with his life? Hard to believe that South Korea.....— Donald J. Trump (@realDonaldTrump) July 4, 2017
....and Japan will put up with this much longer. Perhaps China will put a heavy move on North Korea and end this nonsense once and for all!— Donald J. Trump (@realDonaldTrump) July 4, 2017
Japanese Prime Minister Shinzo Abe earlier said the latest launch showed "North Korea's threat has increased".
He also called for China and Russia to take more "constructive measures" to address this issue.
Earlier this week, North Korea was a key topic in phone calls between Mr Trump and the leaders of China and Japan.