A powerful blast was heard in Nalchik, the capital of the North Caucasus Republic of Kabardino-Balkaria, a police source said on Friday, RIA Novosti reported.
The blast was heard at approximately 23:58 Moscow time [20:58 GMT] on Thursday.
"The blast occurred when a police car was passing by," the source said. "The explosive device was probably planted under a manhole cover."
According to preliminary reports, no one was injured.
While Chechnya, under the leadership of former-separatist-fighter-turned-Kremlin-ally Ramzan Kadyrov, has seen a marked reduction in militant attacks in recent years, violence has erupted in the neighboring republics of Ingushetia, Dagestan and Kabardino-Balkaria.