Nabhan, whose nationality is not disclosed, worked for the Belgian Enabel agency, assisting small businesses
"We have already repeatedly stated that our country does not intend to attack anyone. We consider even the thought of a war between our people to be unacceptable," said Alexei Zaitsev, a spokesman for the ministry.
Russia, which seized Crimea from Ukraine in 2014 and has backed an insurgency in eastern Ukraine, has built up forces on its territory near Ukraine as well as in neighbouring Belarus.
Kyiv rejects Russia's version that the separatist conflict in eastern Ukraine is a civil war that has nothing to do with Moscow, saying Russia is supporting the separatists with covert forces on the ground.