Resolving the North Korean nuclear crisis is impossible with sanctions and pressure alone, Russian President Vladimir Putin said on Wednesday after meeting his South Korean counterpart, adding that the impact of cutting oil would be worrying.

Putin met South Korea’s Moon Jae-in on the sidelines of an economic summit in the eastern Russian city of Vladivostok amid mounting international concern that their neighbor plans more weapons tests, possibly a long-range missile launch ahead of a weekend anniversary.

Putin denounced North Korea’s sixth and largest nuclear bomb test on Sunday, saying Russia did not recognize its nuclear status.

“Pyongyang’s missile and nuclear program is a crude violation of U.N. Security Council resolutions, undermines the non-proliferation regime and creates a threat to the security of northeastern Asia,” Putin said at a joint news conference.

“At the same time, it is clear that it is impossible to resolve the problem of the Korean peninsula only by sanctions and pressure,” he said.

No headway could be made without political and diplomatic tools, Putin said, later telling the TASS news agency that Russian and North Korean delegations might meet at the Vladivostok forum.

Moon, who came to power this year advocating a policy of pursuing engagement with North Korea, has come under increasing pressure to take a harder line.

He has asked the United Nations to consider tough new sanctions after North Korea’s latest nuclear test.

Diplomats say the U.N. Security Council could consider banning North Korean textile exports, barring its airline or stopping supplies of oil to the government and military.

Other measures could include preventing North Koreans from working abroad and putting top officials on a blacklist aimed at imposing asset freezes and travel bans.

“I ask Russia to actively cooperate as this time it is inevitable that North Korea’s oil supply should be cut at the least,” Moon told Putin, according to a readout from a South Korean official.