Greece and Cyprus have urged the European Union to take punitive measures against Turkey amid escalating tensions in the eastern Mediterranean over offshore energy reserves.

As Ankara upped the ante by announcing it would expand exploration for potentially lucrative gas resources in the region, the Greek prime minister, Alexis Tsipras, appealed to the EU “to unreservedly condemn the illegal actions of Turkey”.

The appeal came days after a state-of the-art Turkish drilling vessel, the Fatih, launched offshore operations in waters considered part of Cyprus’s exclusive economic zone despite warnings from western allies.

A statement released on Tuesday by Tsipras’s office following a telephone conversation between the Greek leader and Donald Tusk, the European council’s president, underscored the severity of a situation that is causing growing alarm among diplomats in Athens, Ankara and Nicosia.

Raising the spectre of sanctions, it said: “The prime minister stressed that the European council should examine specific measures against those involved in these illegal activities if Turkey insists on violating international law.”

Nicosia also threatened to veto EU enlargement talks if the bloc failed to take action against Ankara.

Cyprus and Turkey have been increasingly at odd over natural gas deposits believed to lie deep in the seabed of the eastern Mediterranean. Last week, the government of the Cypriot president, Nicos Anastasiades , issued arrest warrants for the crew of the Fatih, accusing the ship of infringing on the island republic’s sovereign territory.

Senior Turkish diplomats hit back, saying Greek Cypriots had not only “crossed the line” but were “playing with fire”.”

Turkey’s president, Recep Tayyip Erdoğan, has taken an increasingly strident stance on an issue he believes will play well at a time when the Turkish economy is in freefall and his own authority increasingly questioned in the wake of municipal elections earlier this year.

Despite expressions of consternation in Washington and other western capitals, the Turkish leader has warned against foreign oil companies participating in what has become an all-out race to tap underwater hydrocarbons.

In a move seen as a brazen act of brinkmanship, Turkey dispatched gunboats to prevent the Italian energy company ENI from conducting drilling operations commissioned by the Cypriot government last year. The company ultimately abandoned the search.

The standoff has highlighted the fragility of Cyprus, the EU’s most easterly member and the bloc’s only divided state. As regional powerhouse, Ankara has long argued that areas of Cyprus’s maritime zone, known formally as its exclusive economic zone, fall within the jurisdiction of Turkey and the war-partitioned island’s Turkish Cypriot community.

Since 1974, when Cyprus was split permanently along ethnic lines after Turkey invaded in the wake of an abortive attempt at union with Greece, Ankara has refused to acknowledge the internationally recognised Greek Cypriot south.

On Sunday, as the war of words escalated following a EU summit meeting that chastised Ankara for its actions, the Turkish foreign ministry hit back, saying: “These [expressions of support] do not have any value, meaning or effect for us.”

The Greek Cypriot government in Nicosia, backed by other EU members, has repeatedly claimed it is Cyprus’s sovereign right to drill in waters around the island. Unlike Turkey and Greece, which are both Nato members, Cyprus has no navy but has launched a far-reaching diplomatic initiative to shore up support among fellow EU states.

Brussels has watched the sabre-rattling with mounting alarm. The French president, Emmanuel Macron, told his EU counterparts at the weekend: “The EU will not show any weakness on this subject.”

But on Tuesday, the Turkish government ratcheted up the pressure further, announcing that a second ship, the Yavuz, will be sent to the region later this week.

Speaking to the country’s state-run news Anadolu news agency, Ankara’s energy minister, Fatih Dönmez, vowed that exploration would be expanded in the coming weeks. “We are now at around 3,000 meters deep. We target to drill to around 5,000 to 5,500 metres deep from sea level. We have 100 to 120 days … for this task. We will have reached our targeted point at the end of July.”