Croatia is signing its long-awaited accession treaty that will bring it into the European Union in 2013 after ratification by the legislatures of the bloc's 27 member nations.
EU president Herman Van Rompuy welcomed the move, saying Croatia will be an "active observer" in all EU forums until it becomes a full member.
Croatia's entry talks lasted seven years and were held up repeatedly due to territorial disputes with neighbouring Slovenia and demands that it arrest remaining war crimes suspects.
Officials said EU leaders may postpone making Serbia a candidate for the bloc until their next summit in March.
The officials said Montenegro was likely to receive the green light to open accession negotiations next June.