An explosion was reported in the United States embassy compound in Ukraine's capital Kiev on Thursday morning, police said.
The authorities said that no one has been reported injured in the incident.
The Ukrainian police said that an attacker threw an explosive device over the embassy's fence just after midnight.
"Investigators found that an unknown person threw an explosive device on the territory of the diplomatic mission," a police statement said.
The authorities are calling the incident a terrorist act.
More details are awaited.