ZHYTOMYR, Ukraine — The North Korean spy, posing as a member of his country’s trade delegation in Belarus, thought he was photographing a secret scientific report on missile technology as he snapped away with a small camera in a dingy garage in the Ukrainian city of Dnipro, the home of Ukraine’s Soviet-era rocket industry.

But the report he took pictures of was a fake, part of a sting operation mounted by Ukraine’s security service to prevent the leak of missile secrets, and now the spy, 56-year-old Ri Tae-gil, is in prison convicted of espionage. He sleeps on the bottom bunk in a cell shared with eight Ukrainian inmates, four of them convicted murderers.

His July 2011 arrest in eastern Ukraine along with a fellow North Korean spy, which was first reported earlier this month by CNN, shows the extent to which Pyongyang has scoured the world for foreign technology to reinvigorate what had been a faltering program to develop long-range missiles.

Mr. Ri and his partner, Ryu Song-chol, 46, were arrested just a few days before North Korea’s then leader, Kim Jong-il, announced during a visit to Russia that Pyongyang was considering a moratorium on missile production and nuclear weapons testing. The moratorium never came about.

Instead, following Mr. Kim’s death in December 2011 and the ascent to power of his son Kim Jong-un, Pyongyang ramped up both its nuclear and missile programs. In July this year, North Korea for the first time launched missiles that experts said were capable of hitting the United States.

While speaking good Russian, which is widely spoken in Ukraine, Mr. Ri does not talk much, his cellmates said, but he does watch a lot of television, particularly reports on the accelerating progress of a North Korean missile program that he had tried in vain to serve. Instead of prized secrets, he received an eight-year prison term for espionage.

Mr. Ri’s prison cell. According to his cellmates, Mr. Ri does not talk much, but does watch a lot of TV, particularly reports on the progress of the North Korean missile program. Credit Brendan Hoffman for The New York Times

His fourth floor cell at the No. 8 Prison in Zhytomyr, a hulking brick building built during the reign of Russia’s last czar, has cable TV. A brick wall in the courtyard below offers a grim reminder of less accommodating times: it is pockmarked with bullet holes left by Stalin’s secret police, the NKVD, later renamed the KGB, which used the spot to execute prisoners.

Asked in an interview whether he felt pride at North Korea’s recent string of successful launches, Mr. Ri, who has a family back in Pyongyang, blanched and said he did not want to talk about rockets.

Apparently eager to counter a widespread belief that North Korea had made such fast progress by stealing foreign technology, he said that his country “has had good rockets for more than 20 years” and also very good engineers.

Denys Chernyshov, Ukraine’s deputy minister of justice, described Mr. Ri and Mr. Ryu as “very well trained. They are tough guys, real spies,” he said.

He noted that, despite six years of detention, they had never written or received letters from family or friends. “They live in a total vacuum,” he said.

After a string of failed tests with an intermediate range missile, Musudan, that had been the target of American sabotage, North Korea last year suddenly and mysteriously found success. It began with the rollout of a new missile in September that not only worked but also demonstrated a capability to travel ever further distances in a series of tests since then. In July, Pyongyang launched a missile capable of reaching the United States.

Seeking to explain North Korea’s mysterious success, some experts have pointed a finger at Ukraine, particularly the Yuzhmash rocket factory and its Yuzhnoye design bureau in Dnipro, the town where Mr. Ri and Mr. Ryu were arrested.

Ukraine’s president, Petro O. Poroshenko, at the Yuzhmash missile factory in Dnipro in 2014. Some experts say North Korea owes its advances in missile design to classified materials obtained from Yuzmash and its Yuzhnoye design bureau in Dnipro. Credit Pool photo by Mykhailo Markiv

Ukraine has adamantly denied allowing leaks of missile technology, pointing to the arrest and conviction of the two spies as proof that the country is capable of combating North Koreans hunting for missile secrets. Ukrainian rockets mostly date to Soviet times but are still far more advanced than anything Pyongyang had until recently been able to produce.

Oleksandr Turchynov, the head of Ukraine’s security and defense council, said it was “completely impossible” that North Korea obtained either missile engines or their design documents from Ukraine.

The RD-250, a Soviet-era rocket engine that some experts say resembles engines used in recent North Korean launches, used to be produced in Dnipro, Mr. Turchynov said, but production stopped in 1991. The entire production line, he said, was dismantled in 1994. Moreover, he added, Ukraine never made whole engines but only supplied parts to the RD-250’s main manufacturer, Energomash, in Russia.