* Turkish research vessel, frigate south of Cyprus coast
* Cypriot govt says research attempt violates sovereignty
* Turkey doesn't recognise Greek Cypriot claim in area
By Michele Kambas
NICOSIA, Oct 20 (Reuters) - Cyprus said on Monday a Turkish research vessel had encroached on an area off its southern coast where it is searching for gas, and it was preparing counter-measures in an escalating row over rights to hydrocarbons in the eastern Mediterranean.
Cyprus, a member of the European Union, has become particularly keen to develop offshore gas reserves as a potential source of revenue since it was compelled to seek an international financial bailout in early 2013.
Greek Cypriot authorities said the Turkish vessel Barbaros, which collects seismic data, entered their exclusive economic zone on Monday morning and intended to stay in the area according to a maritime advisory issued in early October.
"This is a clear violation of Cypriot sovereign rights," said Nicos Christodoulides, the Cypriot government spokesman.
He described it as the "most serious" escalation of a dispute that has long simmered since the declaration of a breakaway Turkish Cypriot state in north Cyprus in 1983.
There was no immediate response from Turkish authorities. Turkey, which backs the Turkish Cypriot entity, does not recognise the jurisdiction of the Nicosia government in the exploration area off the southeastern tip of Cyprus.
The row has already triggered a suspension of peace talks between Greek and Turkish Cypriots on the island partitioned by a 1974 Turkish invasion that followed a brief coup engineered by the military junta then ruling Greece.
"When the reasons we cited for suspending participation in the talks cease to exist, we will go back to negotiations," Christodoulides said.
He said Cyprus would probably announce on Tuesday a series of counter-measures. They would be "political, diplomatic and legal," Christodoulides said, without elaborating. He did say, however, that they would not involve the closure of any checkpoints along the U.N.-monitored ceasefire line between the Greek and Turkish Cypriot sides of the island.
Greek Cypriots running Cyprus's internationally recognised government see the presence of the vessel as a direct slight to its sovereignty, since it has already licensed parts of its sea zone to multinationals to search for gas. Italy's ENI has a drilling rig in the area.
Although there have been disputes over Cyprus's attempts to tap offshore hydrocarbon wealth before, Cypriot officials said this was the first time a foreign vessel with a clear mandate to carry out seismic research had been deployed.
A Cypriot defence ministry source said that in addition to the Barbaros, two support vessels and a Turkish navy frigate were about 20 miles south of Cape Greco, the easternmost part of Cyprus's southern coast.
A U.S. company, Noble, found an estimated 5 trillion cubic metres of gas south of Cyprus in late 2011, straddling a median line with Israel where major discoveries have been made in the past decade.
France's Total TOTf.PA has also signed a concession to drill for gas with the Nicosia government.
Greek Cypriots say any hydrocarbon wealth discovered will be shared with the Turkish Cypriots in the event of a peace deal. (Editing by Mark Heinrich)