New Delhi, June 30 -- A blast from an explosive device has seriously injured three people at a residential building in Monaco, and the attacker fled to France, local authorities said. French and Ukrainian media reported that a Ukrainian magnate and his family were those injured.

The unusual attack Monday night stunned the elite principality on the Mediterranean Coast. Monaco's leader Prince Albert II called it "an odious act" and said all the country's services were mobilized to ensure security.

French and Monaco authorities are searching for the attacker, whose motive is under investigation, Monaco's most senior government official, Minister of State Christophe Mirmand, told reporters.

The explosion occurred around 9 p.m. at the entry...