WASHINGTON — Kim Jong-un is headed to the moon.

That, at least, is one of the official North Korean explanations for the testing last week of a rocket engine that, if as powerful as the North claims, would rival the commercial rockets that Jeff Bezos and Elon Musk, of Amazon and Tesla, now use in their aerospace companies to fire payloads into space.

Inside the United States’ intelligence agencies, though, there is considerable skepticism that North Korea is truly eager to plant a flag on the lunar landscape. The agencies are exploring another explanation: that Mr. Kim, the North Korean leader, is racing ahead, as the United States is distracted by a bruising presidential election, to develop a way for his growing arsenal of nuclear weapons to reach New York and Washington.

The North may not be working alone. An intelligence finding that the United States quietly made public in January suggests that the development of the North’s big engine, which it claims produces 80 tons of thrust, may be part of a joint partnership with Iran. A Treasury Department announcement of sanctions against Iranian officials and engineers named two who had “traveled to North Korea to work on an 80-ton rocket booster being developed by the North Korean government.”

The Obama administration has responded to North Korea’s nuclear and missile tests with gradually escalating sanctions, mostly through the United Nations. But on Monday it went a step farther, announcing criminal charges and Treasury Department sanctions against four Chinese individuals and a company that it said engaged in money laundering to help the North’s programs for weapons of mass destruction.