According to the Turkish daily Zaman, Ankara said it would block EU participation at the summit unless the Organisation of the Islamic Cooperation (OIC) is also allowed in the room. NATO leaders are due to meet in Chicago 20-21 May.
The stand-off appears as an embarrassment for the alliance because European Council President Herman Van Rompuy and European Commission President José Manuel Barroso have already received invitations to attend the opening dinner, reports say.
The EU as an entity needs an invitation to attend NATO events. Turkey, a NATO member since 1952, has the power to veto the EU's participation.
The EU has acquired new political weight with the Lisbon Treaty. It is also participating in efforts for the reconstruction and democratisation of Afghanistan, and its role is expected to grow after NATO's security operation ends in 2014.
The Turkish daily Hürriyet quoted an unnamed Turkish official saying that Ankara did not name the OIC as a condition for the EU's participation.
But the source seemed to confirm that Ankara would oppose an EU participation in Chicago.
"NATO should not waste time trying to invite more representatives from outside organisations," the source was quoted as saying. The source also praised the OIC, a bloc of 57 countries, for helping NATO in Libya "to no lesser extent than the EU".
Turkey claims that until the Republic of Cyprus, which it doesn't recognise (see background), became an EU member in 2004, its NATO relations had been going quite well. Ankara has since then been preventing NATO from exchanging information with the EU on the grounds that Cyprus, a non-aligned country, is not a member of the Partnership for Peace – a NATO programme seen as stepping stone to alliance membership.
Cyprus is to assume the six-month rotating EU presidency on July. Turkey has warned it would freeze relations with the Union during this period, and that it would consider annexing northern Cyprus, which is technically EU territory.
Turkey reportedly has also blocked Israel’s participation at the Chicago summit, highlighting its determination to prevent its new foe from cooperating with the alliance following the Israeli attack in May 2010 on a flotilla carrying aid and pro-Palestinian activists to the Gaza Strip. Nine Turkish civilians were killed in the attack.
Turkey insists that NATO-Israel relations cannot be restored until Turkish-Israeli relations are normalised.