Athens - Greece looks for EU support this week in tough upcoming talks with Turkey after securing a diplomatic boost from the visiting US secretary of state.

An ongoing crisis over maritime borders and energy exploration is to be discussed at an EU leaders' summit Thursday and Friday in Brussels.

European Council president Charles Michel warned Tuesday that "all options" including sanctions were still on the table if Turkey did not engage "constructively" in attempts to ease tension.

Athens and Ankara have said they are prepared to resume talks on continental shelf disputes interrupted in 2016.

"The time for diplomacy has come," Greek Prime Minister Kyriakos Mitsotakis told his cabinet on Wednesday, after a two-day visit by US Secretary of State Mike Pompeo.

Turkey sent a research vessel to disputed waters between Cyprus and the Greek islands of Kastellorizo and Crete

"It is up to our neighbours to really demonstrate if they sincerely desire de-escalation and dialogue."

Tension spiked dangerously last month when Turkey sent a research vessel escorted by warships to disputed waters between Cyprus and the Greek islands of Kastellorizo and Crete.

Greece sent its warships in response, and received strong backing from France, raising fears of a row within NATO.

Mitsotakis then unveiled Greece's most ambitious arms spending programme in two decades, including 15,000 additional troops, frigates, missiles and French-made warplanes.

The talks are to be held in Istanbul but a date has not been announced.

Sixty prior attempts have been held since 2002 without success.

Pompeo on Tuesday said the US "strongly" supported dialogue -- and delighted his hosts by announcing that the US navy's newest expeditionary sea base would from now on be based at Souda Bay on Crete.

- Expectations 'limited' -

With Ankara engaged in a "revisionist neo-Ottoman thrust", expectations of a breakthrough are limited, says Kostas Lavdas, professor of European politics at Athens' Panteion university.

The lack of agreed maritime borders between Turkey and Greece, or Turkey and Cyprus, has turned into a broader problem in the past decade following the discovery of major hydrocarbon reserves in the eastern Mediterranean.

Last year Cyprus, Greece, Egypt, Israel, Jordan, Italy and the Palestinian territories agreed to create an "East Mediterranean Gas Forum" without Turkey.

US strongly supports Greece-Turkey dialogue: Pompeo

Turkey in November responded with a maritime deal with the UN-recognised government in Libya, and Greece in August brokered a maritime agreement with Egypt.

"Turkey spasmodically seeks a political victory," notes Tassos Giannitsis, Greek minister for European affairs in 2001-2004.

- International arbitration -

"Seeing other countries in the eastern Mediterranean acquiring new economic potential, Turkey seeks to muscle in wherever it can, and dampen any possible gains by Cyprus."

"(It) seeks to satisfy its regional geopolitical aims, regardless of whether they are compatible with international rules. This sets a dangerous example in international relations," Giannitsis told AFP.

Ankara argues that it has the largest coastline of all the eastern Mediterranean nations but a disproportionately small share of the sea because of Greece's far-flung islands -- some of them within sight of Turkey's shore.

Athens says its claims are grounded in postwar treaties, some of them signed by Ankara.

If the two sides cannot agree, Mitsotakis says the dispute should be put before the International Court of Justice at The Hague.

"Assuming that Turkey were to change course and finally accept the Hague as an option, there would be some advantages -- as well as the need to possibly compromise on particular issues -- in the sense that frictions have lasted too long and Turkey is becoming more, not less aggressive," said Lavdas.

Mitsotakis "understands that the costs of inertia and non-resolution are substantially higher than thought," says Ioannis Grigoriadis, senior research fellow at the Hellenic Foundation of European and Foreign Policy and head of its Turkey programme.

"The court might not accept 100 percent of Greek positions, but the resolution of the conflict would be a big boost for Greece, Turkey and the region," he said.

Turkish President Recep Tayyip Erdogan has sought to galvanise his more conservative and nationalist supporters at a time when Turkey is suffering under a new spell of inflation and economic uncertainty caused by the coronavirus.

In a speech to the United Nations on September 22, Erdogan said Turkey was defending itself against a "winner-takes-all" mentality displayed by regional rivals.

"The reason for the problems existing in the region today is the one-sided steps taken by Greece and the Greek Cypriots since 2003 with maximalist demands," the Turkish leader said.

- 'Maximalist' demands -

Greece remains suspicious of Turkish overtures after a barrage of aggressive rhetoric from Erdogan and other Ankara officials in past weeks.

Erdogan in August called the leaders of Greece and France "greedy and incompetent" for challenging Turkish energy exploration in the eastern Mediterranean.

"When the time comes to fight, we will not hesitate to make sacrifices," he added in September.

"The question is: when they stand against us in the Mediterranean, are they ready to make the same sacrifices?" Erdogan said.

"In recent years," says Lavdas, "almost everyone in the region feels threatened by Turkey in some way or another: Greece, Cyprus, Egypt, Israel."

"The need to achieve an improved balance of forces in the eastern Mediterranean has always been important but it now appears to be urgent."

By John Hadoulis