SEOUL, South Korea — North Korea has started rebuilding the facilities it uses to launch satellites into orbit and test engines and other technologies for its intercontinental ballistic missile program, according to American military analysts and South Korean intelligence officials.

The revelation comes days after the breakdown of the second summit meeting between the North’s leader, Kim Jong-un, and President Trump last week in Hanoi, Vietnam. It could be a first sign that North Korea is preparing to end its moratorium on missile tests, which Mr. Trump has claimed as a major diplomatic achievement.

North Korea began dismantling the Sohae Satellite Launching Station in Tongchang-ri near its northwestern border with China last summer, after Mr. Kim held his first meeting with Mr. Trump in June in Singapore. It partially took down an engine test site, a rocket launchpad and a rail-mounted building used by engineers to assemble launch vehicles and move them to the launchpad.

The North did not completely dismantle the facilities, and when Mr. Kim met with President Moon Jae-in of South Korea in September, he offered to destroy them in the presence of American experts.

Reports published Tuesday on the rebuilding at Tongchang-ri were based on satellite images obtained Saturday, but analysts said the work could have begun as early as mid-February.

“Based on commercial satellite imagery, efforts to rebuild these structures started sometime between February 16 and March 2, 2019,” 38 North, a website specializing in North Korea analysis, said in a report about the Tongchang-ri facilities on Tuesday.

“On the launchpad, the rail-mounted transfer building is being reassembled,” it said. “At the engine test stand, it appears that the engine support structure is being reassembled.”

Beyond Parallel, a website run by the Washington-based Center for Strategic and International Studies, published a report with similar assessments on Tuesday.

“Commercial satellite imagery acquired on March 2, 2019, shows that North Korea is pursuing a rapid rebuilding of the long-range rocket site,” it said. The renewed activity “may indicate North Korean plans to demonstrate resolve” after the Hanoi summit, it said.

Officially, North Korea says it no longer needs to carry out nuclear or missile tests because it has finished developing its nuclear-capable intercontinental ballistic missiles and begun mass-producing them. But some Western officials and analysts still doubt that the country has mastered the technologies needed to reliably strike a target across an ocean with a missile.

In his Singapore meeting with Mr. Trump, Mr. Kim made a vague commitment to “work toward complete denuclearization of the Korean Peninsula.” But the North has since balked at taking specific actions toward dismantling its nuclear and missile programs, criticizing what it called Washington’s “unilateral, gangster-like demand for denuclearization” and insisting that it will not move toward denuclearization unless the United States takes “corresponding” steps.