Tension between Greece and Turkey has escalated in the south Aegean following Turkey’s decision to send seismic survey vessel Oruc Reis to conduct research in an area of the Greek continental shelf. The vessel is accompanied by two support research vessels and several Turkish warships, according to a statement by Turkey’s Ministry of Defence.

The search for potentially rich oil and gas deposits is to take place south of Kastellorizo, 20 miles into Greece’s continental shelf.

Cables have already been placed in the sea area by the Turkish vessel which had also entered the area last month, but had suspended its voyage amid international concern.

A number of Greek warships are sailing near the area with a message being transmitted at a constant 15-minute interval to the captain of the Oruc Reis vessel to inform both the captain and crew that they have entered the Greek continental shelf and urging them to leave the area.

Greek government sources say that the noise in the area due to the large concentration of warships would not allow the Oruc Reis to carry out work.

Greece’s Foreign Ministry said the mission was a “new serious escalation” of tension and urged Ankara to immediately cease all its illegal activities in the area, but Turkey dismissed Greek objections stating that they were not grounded on legal basis. Reuters reported that Turkey said military presence in the area, for which it gave no details, was purely defensive.

READ MORE: Reactions to Turkey’s NAVTEX to carry out seismic survey around Kastellorizo

Greece’s Prime Minister Kyriakos Mitsotakis called a meeting with military chiefs on Monday after Turkey issued a NAVTEX, an international maritime alert.

Turkey’s mission challenges a Greek-Egyptian gas exploration deal which was signed last Thursday to create an exclusive economic zone (EEZ) in waters containing oil and gas reserves. This deal conflicts with a rival agreement between Turkey and Libya which have sought to establish a similar zone. These zones can extend for 200 miles.

There are also tensions around rival exploration rights around Cyprus which exclude the Turkish-controlled area of the island. Last year Turkey sent two drilling ships, the Fatih and the Yavus, to waters off Cyprus.

READ MORE: Tensions mount at Greek border with Turkey amid contested history of migration in the Aegean

US State Department calls on Turkey to stop raising tension in the area

The US State Department spokesperson said on Monday that his country “is aware that Turkey has issued a notification to other ships of survey activity in the Eastern Mediterranean” and urged Turkey to “halt any such plans for operations and to avoid steps that raise tensions in the region.”

“The United States is deeply concerned by Turkey’s stated plans to survey for natural resources in areas over which Greece and Cyprus assert jurisdiction in the Eastern Mediterranean. Such actions are provocative and raise tensions in the region. Resource development in the Eastern Mediterranean should promote cooperation and provide a foundation for durable energy security and economic prosperity throughout the region,” he said.