Dozens of people have been injured in clashes between South Korean protesters and police as the US military added more launchers to the missile-defence system it installed in a southern town to handle North Korean threats.
Thousands of police officers in riot gear swarmed 400 protesters who had been occupying a road in the rural town of Seongju, South Korea.
Six police officers and 32 other people were injured, none seriously, in the clashes, said a fire department official in Seongju.
Seoul has hardened its stance against Pyongyang after its torrent of arms tests, the latest on Sunday being a detonation of what North Korea said was a thermonuclear weapon built for missiles capable of reaching the US mainland.
The clashes came as South Korean President Moon Jae-in and Japanese Prime Minster Shinzo Abe met in Russia's Far East and repeated their calls for stronger punishment of North Korea over its nuclear ambitions, including denying the country oil supplies.
The demand contradicted the stance of their host, Russian President Vladimir Putin, who has dismissed sanctions as a solution.
Mr Putin said he believes US President Donald Trump's administration is willing to defuse tensions over North Korea's nuclear ambitions. Its sixth nuclear test on Sunday prompted the US to speak about a "military response" and South Korea to conduct major military exercises.
The Russian leader said there are "many reasonable people in the current (US) administration" who are experienced and who have dealt with similar crises.
He called on all North Korea's neighbours to show restraint, suggesting the bellicose rhetoric and military drills are "playing into their hands".
Mr Moon and Mr Abe agreed to co-operate on seeking tougher UN sanctions against North Korea and pledged to strengthen efforts to persuade Beijing and Moscow to cut off oil supplies to the North, said Mr Moon's press secretary.
Mr Putin expressed concern that cutting off oil supplies would hurt regular North Koreans, the official said.
"We should not give in to emotions and push Pyongyang into a corner," Mr Putin said in a news conference after meeting Mr Moon.
Chinese foreign minister Wang Yi said Beijing supports further UN action against North Korea but also wants to see renewed efforts to begin dialogue involving all sides.
China hopes North Korea will "see the situation clearly and come to the right judgment and choice", Mr Wang said.
He said the UN should take "necessary measures", but added that sanctions and pressure should spur negotiation between the sides towards the goal of a peaceful solution on the Korean peninsula.
China is a veto-wielding permanent member of the UN Security Council, as well as North Korea's main trading partner and source of food and fuel aid.
Chinese foreign ministry spokesman Geng Shuang also reiterated Beijing's opposition to South Korea's deployment of the US Terminal High-Altitude Area Defence System, also known as THAAD, which is intended to protect against North Korean missile attacks.
Beijing says the system's powerful radars will be able to monitor flights and missile launches deep inside north-eastern China.
Nato's secretary-general said North Korean behaviour is a global threat and called for a united response.
Jens Stoltenberg said Pyongyang must abandon its nuclear and missile programmes and refrain from further testing.
The European Union's foreign policy chief said the world should not "enter this spiral of a military confrontation that could be extremely dangerous not only for the region but for the entire world".
Federica Mogherini said a demilitarisation of the Korean peninsula should be achieved peacefully through dialogue and diplomacy.