The EU and US have urged Turkey to stop drilling for gas in Cyprus' maritime zone - but Ankara has told its allies it would do so anyway.

Brussels and Washington spoke out over the weekend after a Turkish ship called Fatih, meaning "Conqueror", began drilling a 5,500-metre well in the Mediterranean Sea on Friday (3 May) in a zone claimed both by Cyprus and by the Turkey-backed, breakaway Turkish Republic of Northern Cyprus (TRNC).

Student or retired? Then this plan is for you.

"We urgently call on Turkey to ... refrain from any such illegal action to which the European Union will respond appropriately and in full solidarity with Cyprus," the EU foreign service said on Saturday.

"This step is highly provocative and risks raising tensions in the region. We urge Turkish authorities to halt these operations," the US state department said the following day.

Cyprus, Greece, as well as Egypt issued similar statements.

"This provocative action by Turkey constitutes a flagrant violation of the sovereign rights of the Republic of Cyprus," its foreign ministry said.

But the Turkish government said it would go ahead despite the complaints.

"We will protect our own rights and interests within our continental shelf, as well as those of the Turkish Cypriots," the Turkish foreign ministry said in reaction to the EU on Saturday.

"Turkey is determined to always protect the rights of the people of Turkish Republic of Northern Cyprus ... and not to allow the fait accompli," Turkish defence minister Hulusi Akar told Anadolu, a Turkish state news agency, on Sunday.

The maritime dispute comes amid Turkey's wider drift away from the EU and Nato toward authoritarianism and Russia.

Turkish president Recep Tayyip Erdogan has scorned EU appeals to stop jailing and torturing his political opponents.

He is also building a Russian gas pipeline that will undermine EU energy security and buying a Russian air-defence system despite US warnings that that could compromise its Nato ties.

Turkey would buy the Russian 'S-400' system even if the US imposed sanctions in return, Turkish vice president Fuat Oktay told Kanal 7, a Turkish state broadcaster, on Sunday.

"We've completed the S-400 deal. It's a done deal," Turkish foreign minister Mevlut Cavusoglu also said on Friday while visiting Hungary.

The Turkish Republic of North Cyprus (TRNC) broke away from Cyprus in 1974 after Turkey invaded the island in response to a Greek-sponsored coup.

Turkey continues to keep thousands of troops there and is the only country that recognises TRNC sovereignty.

Cyprus has also tried to drill exploratory wells in what it and the rest of the world, except Turkey, say is its own maritime zone, but Turkish warships scared off its vessels the last time it tried to do so in February last year.

The state owned Turkish Petroleum firm said on Friday it had bought a second drill-ship, called the Yavuz, that is to work alongside the Fatih, boding ill for the dispute.