LAPPEENRANTA, Finland (Reuters) -- European Union retaliation may be limited if Turkey fails to meet an obligation to open its ports to ships from Cyprus this year, EU president Finland signalled on Saturday.

Some diplomats have suggested a Turkish refusal to implement an agreement extending its customs union to all 10 new EU members, including Cyprus, could lead to a complete breakdown of Ankara's negotiations to join the 25-nation bloc.

But Finnish Foreign Minister Erkki Tuomioja, speaking after a meeting of EU foreign ministers in eastern Finland, indicated the response might be confined to a few areas of the talks.

"At the minimum it is understandable that we cannot continue membership negotiations on any items pertaining to the single market," he told a news conference when asked what the consequences of a Turkish refusal would be.

Diplomats said that could affect up to a dozen of the 35 policy areas into which EU law is divided, but would not prevent talks on areas such as education, culture or health.

Turkey says it will only open its ports if the EU makes good on a pledge to end the economic isolation of Turkish Cypriot northern Cyprus, a move which the Greek Cypriot government in Nicosia is blocking. The EU rejects such linkage.

Tuomioja said if Turkey did not ratify the so-called additional protocol, it would create a serious situation.

"But having said this, we still hope to avoid any crisis in our relations. It is not inevitable," he said.

The EU began membership talks with Ankara last year despite strong public opposition in core west European countries, but tension has grown since, ahead of elections in Turkey and France next year.

Diplomats said the EU would make intensive efforts in the next few weeks to try to unblock the issue of direct trade with northern Cyprus, but Nicosia had resisted all pressure so far.

Turkey, which invaded northern Cyprus in 1974 in response to a coup engineered by Greece's then ruling military junta, does not recognise the Greek Cypriot Nicosia government.