"Turkey needs to commit unequivocally to good neighbourly relations and avoid any kind of source of friction, threat, or action directed against a member state," a European Commission spokeswoman said in Brussels.
She insisted that Turkey's behaviour "damages good neighbourly relations and the peaceful settlement of disputes."
"The EU stresses the need to respect the sovereignty of member states over their territorial sea and airspace," she added, also with reference to regular Turkish incursions in Greek sea and airspace.
Tensions increased last Friday after Turkish vessels stopped the Siam 12000, a floating rig belonging to Italian firm ENI. The Saipem 12000 was told not to enter the zone where it was supposed to drill, "because there would be military activities in the destination area," an ENI spokesman said, adding that the rig "prudently executed the orders."
Three days before, the Turkish navy had reserved - for military training - an area in the Mediterranean sea that included part of Cyprus' exclusive economic zone (EEZ).
Under UN rules, EEZ is a sea zone where, under UN rules, countries have special rights for economic activities.
But Turkey does not accept Cyprus' EEZ, saying that it does not take into account the interests of the northern part of Cyprus. Turkey occupied the north of Cyprus militarily and is the only country to recognise it as a state.
The dispute is all the more acute, given that the EEZ is thought to contain important reserves of gas and oil.
On Sunday, the Turkish government denounced Cyprus' "unilateral" drilling, which it said undermined the ''inalienable rights on natural resources of Turkish-Cypriots''. "Turkey's actions are violating international law," Cypriot president Nicos Anastasiades also said on Sunday.
He said that his government would take the "necessary steps," but added that he wanted to "avoid anything which could escalate the crisis."
- The Italian government, for its part, said on Monday that it was monitoring the situation and would take "all possible diplomatic steps to resolve the issue."
- The escalation in Cyprus' EEZ comes a week after Anastasiades was re-elected president of Cyprus, and amid hopes that peace talks could start again between him and Turkish-Cypriot leader Mustafa Akinci, after a previous effort failed last year.
- Turkish vessels regularly patrol in the eastern Mediterranean to put pressure on Cyprus and oil and gas companies.
- But Friday's blocking of the Italian rig is "a step further", a source told EUobserver after the commission's warning to Turkey.
- Turkey's Europe minister, Omer Celik, will meet with EU foreign affairs ministers and EU high representative Federica Mogherini in Sofia later this week.
- The tension over Cyprus is likely to be discussed, within other EU-Turkey issues, but no Mogherini-Celik bilateral discussion has been scheduled yet.
Source: EU Observer