The treaty was signed earlier in the day in Brussels.

The signing of the document has brought to completion one stage on Croatia's road towards Europe and now a second stage has begun, Milanovic said, congratulating all who had contributed to the conclusion of the agreement.

"This is yet another example in which we see that Croatia's road has been a difficult one. Our road in creating the independent state was difficult and long and we paid a high price. Our road towards entry into the EU has also been long and our negotiations have bee longer and tougher than has been the case with other countries," he told a news conference held at the SDP headquarters in Zagreb.

All of this shows that only hard work can help us achieve great results, he added.

Milanovic said that he regarded the signing of the accession treaty as a precondition for a great result, for Croatia to make the most of EU membership while remaining "a normal and loyal member of that supranational alliance".

Milanovic said the EU was currently in a difficult yet not a hopeless situation.

"I believe in Europe as a project and tool that will help Croatia achieve, through its EU membership, its national interests of happiness and prosperity followed by competition and high economic growth," he said.

Asked about the contribution of the government led by Prime Minister Jadranka Kosor to the signing of the document, he said that the outgoing government had certainly made its contribution to today's event, having brought negotiations to a successful completion.

"All governments deserve the credit for it, including the one that successfully concluded the negotiations, and that's the government led by HDZ president Jadranka Kosor," Milanovic said.

Asked about his position on the introduction of the euro in Croatia considering the ongoing crisis in the EU, he said he would be able to offer a better answer when he saw what European leaders had decided on the eurozone.

European leaders have had to make tough decisions for months and this crisis is primarily a crisis of confidence in which difficult political decisions should be made. Europe is not any poorer than it was yesterday or five years ago, its level or prosperity, the know-how and technology is higher than ever. This is a crisis of confidence, he said.

Milanovic said he believed in the prudence and leadership os European leaders, and Croatia would contribute as much as it could, noting that he saw Croatia's admission to the eurozone as a medium-term goal.

He was also asked for a comment on the border arbitration agreement between Croatia and EU member Slovenia, given the fact that procedural timelines specified in the agreement began running with the signing of Croatia's accession treaty today and that he and his party had objected to the adoption of the existing agreement.

Milanovic said that the bilateral agreement included a theoretical possibility for the two countries to again open bilateral negotiations on the matter, "in other words, do everything we have done unsuccessfully in the last 20 years."

He said that the SDP's objections mainly concerned the "unprecedented" method of nomination of arbitrators, but added that the agreement was signed and that his government would abide by it.