The European part of Cyprus has commenced the exploration of an underwater gas field in its disputed territorial waters and immediately - expressly as a retaliatory measure - Turkish prime minister Recep Tayyip Erdogan replied that Turkey would be doing the same off the northern half of the island split in two.

The tension created in the area for at least the past two weeks due to the Turkish-Israeli diplomatic crisis rose after Ankara announced it would be sending warships and planes despite the EU's calls for calm and talks.

Cypriot sources have said that the gas exploration operations in the waters south of the island began yesterday evening, with an acceleration compared with the presidential announcement yesterday that they would be beginning ''over the next few days'', at least as concerns the true ''drilling'' in the Block 12 (also known as Aphrodite) field in the Republic of Cyprus's Exclusive Economic Zone.

In action is the Homer Ferrington oil drilling rig of the Texan company Noble Energy which - after conducting surveys in Israeli territorial waters - has today been positioned on the perpendicular of the field.

Faithful to the threats expressed over the past few days, Erdogan - on his departure for the UN General Assembly - announced that Turkish underwater exploration for gas off the northern coast of Cyprus may begin ''this week'' in ''our exclusive economic zone'', and that it will be monitored by planes, frigates and torpedo boats - rapidly-moving but small, the latter launch torpedoes against larger ships. Cypriot radio has already reported the presence, in international waters but close to the Homer Ferrington rig, Turkish ships that for the moment have not shown any aggressive intentions.

On the other hand, a Turkish newspaper noted yesterday two F-16 fighterplanes streaking through the air with trajectories worthy of an appeal to the UN, and Greece has promised strong support to its neighbouring islanders should an attack take place.

The prospect of war, previously conjured up by Erdogan in speaking at the beginning of the month on naval escorts to flotillas carrying humanitarian aid to Gaza, spurred EU foreign policy chief Catherine Ashton to urge Turkey to ''abstain from any sort of threat'' which could result in even higher tension levels with Cyprus, the island divided since the Turkish invasion in 1974 between one part internationally recognised (the Greek-Cypriot one) and another accredited only by Ankara.

Brussels has appealed to Turkey and Cyprus ''to find a comprehensive solution as soon as possible'' on the status of Northern Cyprus, which is at the centre talks under UN aegis

. The atmosphere is, however, not of one of constructive talks - seeing as a Turkish minister has today reiterated a threat formulated yesterday by the deputy prime minister:

if Cyprus gets the scheduled EU rotating presidency in the second half of next year without previously having solved the issue of the divided island, Turkey will freeze relations with the EU.