The EU expects all western Balkan countries by the end of 2008 to have signed initial deals paving the way for eventual entry into the 27-member bloc.
The EU's Olli Rehn said much must yet be done by the entry hopefuls
Enlargement Commissioner Olli Rehn said Albania, Bosnia-Hercegovina, Croatia, Montenegro, Macedonia and Serbia were on course to fulfil all EU conditions.
He also praised Serbia's co-operation with the UN war crimes tribunal.
But Turkey was urged to improve freedom of expression and minority rights if it wanted its membership bid to progress.
Mr Rehn said the EU and Turkey should not begin critical accession talks on justice and human rights until Ankara had amended Article 301, a law used to prosecute journalists and intellectuals for "insulting Turkishness".
The commissioner said the international community supported Turkey's efforts to protect its people from recent attacks by Kurdish separatist fighters based in northern Iraq, but called on Ankara to refrain from "disproportionate" military action.
"I expect that in 2008 conditions will be fulfilled and thus we will be able to complete Stabilisation and Association Agreements (SAA) with all countries in the region," Rehn said in his annual progress report on candidate countries.
Drafts of the Stabilisation and Association Agreements, which are regarded as the first step towards full EU membership, will be initialled in Brussels on Wednesday.
However, Mr Rehn stressed the SAA with Serbia could only be signed formally when Belgrade co-operates fully with the International Criminal Tribunal for the former Yugoslavia (ICTY) by helping to arrest fugitives such as the Bosnian Serb wartime leaders, Ratko Mladic and Radovan Karadzic.
"This marks a real turning point for Serbia," he told a news conference. "Now Serbia has to go the last mile and achieve full co-operation."
Mr Rehn said he had made the decision after discussions with the UN war crimes prosecutor, Carla del Ponte.
Ms del Ponte "considers there is now the political will and intensified action by the Serbian government to arrest and transfer the remaining fugitives to The Hague tribunal", he said.
The BBC's Nick Hawton in Belgrade says it is relatively rare for Serbia to receive good news from abroad and the initialling of the SAA will be an important signal to the reformers and modernisers within the political establishment that there is a future within Europe.
It will strengthen their hand in the difficult times ahead when the long-term political status of Kosovo is to be determined, an issue which usually benefits the nationalists, our correspondent says.
The breakaway Serbian province, meanwhile, was singled out for criticism by the EU in its annual progress report on membership candidates.
The EU said Kosovo, which is run by the United Nations, was plagued by corruption and described its public administration as weak and inefficient.
"Due to a lack of clear political will to fight corruption, and to insufficient legislative and implementing measures, corruption is still widespread," the report said.
"Civil servants are still vulnerable to political interference, corrupt practices and nepotism," it added.
The report also claimed that war crimes trials were being delayed by the unwillingness of the Kosovan population to testify, and said there was no adequate mechanism to address complaints against the UN administration.