Cyprus says a U.S. firm has begun exploratory drilling off the divided island's southern coast despite strong warnings from Turkey not to do so.
A Cypriot energy official, Solon Kassinis, said drilling for gas began late on September 18.
In reaction, Turkish Prime Minister Recep Tayyip Erdogan said Turkish "frigates, gunboats, and its air force will constantly monitor developments in the area."
He said Turkey would also begin its own oil and gas search as early as this week.
Cyprus has been divided along ethnic lines since 1974, when Turkish forces occupied its northern third in response to a Greece-engineered coup aimed at union with Athens.
Turkey has repeatedly called on Cyprus, which is a European Union member, to postpone its gas exploration, saying it has no right to do so while the island remains split.
compiled from agency reports