The United States is prepared to take “simultaneous steps” with North Korea to achieve peace on the Korean Peninsula, the American ambassador to the United Nations said Wednesday, but she also warned the North Koreans against conducting further missile tests.

The ambassador, Kelly Craft, made the remarks during a meeting of the United Nations Security Council, which was called at her request over worries that North Korea could soon resume testing of its long-range missiles or perhaps even nuclear weapons, which it halted in 2017.

That moratorium was declared by North Korea’s leader, Kim Jong-un, during a period of optimistic-sounding diplomacy, punctuated by a summit meeting with President Trump in Singapore in June 2018, and two more meetings in following months. But talks between the two countries aimed at ending seven decades of hostility, and with it the elimination of Mr. Kim’s nuclear arsenal, have long been stalled.

Mr. Kim, angered over United Nations sanctions that have isolated North Korea for years, has been hinting of a change in policy unless diplomatic progress is made with the United States. His military has launched more than a dozen short-range missiles this year, in what are widely viewed as signs of Mr. Kim’s impatience.