Speaking at a youth forum east of Moscow, Mr. Lavrov, the foreign minister, said Moscow was “very worried” by fiery declarations in Washington and Pyongyang.

“Talk of the need to carry out a pre-emptive strike at North Korea, Pyongyang’s talk of the need to strike at Guam island at the U.S. military base, this has been continual, and we are very worried by this,” Mr. Lavrov said.

He said that Russia, which last weekend joined the United States and China in voting at the United Nations Security Council for severe new sanctions against Pyongyang, did not accept North Korea as a nuclear power. But he said it was up to the United States, as the more powerful country, to take the first step.

“I believe when it actually comes to a fight, the one that is stronger and smarter should take the first step away from the dangerous line,” Mr. Lavrov said. “Together with China, Russia offered a very reasonable plan providing for the ‘double freeze.’ Kim Jong-un freezes any nuclear tests, any ballistic missile launches, while the U.S. and South Korea freeze large-scale military exercises, which are constantly used by North Korea as an excuse to conduct tests.”

Russian experts worry that while the United States is unlikely to launch an all-out military attack on North Korea, it could make a limited strike. This, warned Vladimir I. Batyuk, of the Institute for U.S. and Canadian Studies in Moscow, would shatter the united front that Russia, China and the United States displayed in the sanctions vote at the United Nations.

“If the Americans apply military force against the D.P.R.K., they will clearly lose whatever political support Moscow and Beijing have provided so far for U.S. policy toward North Korea,” Mr. Batyuk told Kommersant, a Russian newspaper, using the initials for the Democratic People’s Republic of Korea, the formal name of the North.

Vladimir Frolov, an independent foreign policy specialist, contrasted Russia’s “passive” response to the risk of nuclear war near its eastern border with the “hyperactive Russian diplomacy on Syria and Ukraine.”