The number of Russian nuclear-monitoring stations that have gone silent has doubled to four, an international arms-control official said, heightening concerns among observers that Russia is attempting to conceal evidence from an explosion at a missile-test site this month.

Russia monitoring stations designed to detect nuclear radiation at the towns of Bilibino and Zalesovo stopped transmitting data on Aug. 13, Lassina Zerbo, the executive director of the Comprehensive Nuclear Test Ban Organization, told The Wall Street Journal on Monday.

Those data disruptions occurred three days after two other monitoring stations, at Dubna and Kirov, which were closer to the site of the Aug. 8 accident, went silent, as the Journal reported on Sunday.

“Experts continue to reach out to our collaborators in Russia to resume operations as expediently as possible,” Mr. Zerbo wrote in an email.

The U.S. and other world powers have a variety of means to monitor military tests in Russia as well as Moscow’s compliance with the Comprehensive Nuclear Test Ban Treaty, which prohibits nuclear tests globally.