New satellite images that show a prolonged and heightened level of activity at North Korea’s underground testing site has led observers to suggest that the country might be planning another nuclear test.

It’s impossible to tell if North Korea is putting on a performance for the satellites — a ploy it has sometimes used to raise tensions — or if a sixth nuclear test is imminent. But analysts agree that North Korea is determined to make progress on its nuclear and missile programs.

“They are trying to get a working arsenal, so the more they test, the more they learn,” said Jon Wolfsthal, a senior nonproliferation adviser in the Obama administration who is now at the Carnegie Endowment for International Peace.

“It’s likely that they’re trying to make a device small enough to achieve their goal of putting a nuclear warhead on a long-range missile,” Wolfsthal said.

North Korea has made no secret of its ambitions to build a missile capable of reaching the continental United States, and Kim Jong Un said in a Jan. 1 address that his regime had “entered the final stage of preparation for a test-launch of an intercontinental ballistic missile.”

It has been launching medium- and intermediate-range missiles and testing rocket engines at a surprisingly fast pace over the past year, tests that analysts view as part of a bigger ICBM programme.