Signing the Energy Community Treaty, the EU and nine partners of Southeast Europe - Croatia, Bosnia-Herzegovina, Serbia, Montenegro, Macedonia, Albania, Romania, Bulgaria and UNMIK on behalf of Kosovo - will create the legal framework for an integrated energy market.
Negotiations with Turkey are ongoing for joining the treaty at a later stage, the EC reported.
Commissioner Andis Piebalgs in charge of energy, who signed the treaty on behalf of the EU, said the "Energy Community Treaty will enhance security of supply and give support to a strategically vital sector".
On behalf of Croatia, the treaty was signed by Economy Minister Branko Vukelic who hailed the treaty as a key document which integrated Croatia with the EU energy system and led the country to the Eu even before the formal admission.
The treaty, Vukelic commented, will ensure Croatia a more stable energy and gas supply and it creates a market for investments into Croatia's energy sector as well as opens foreign markets for investments by Croatian energy companies.
European Commission President Jose Manuel Barroso hailed the Energy Community Treaty as "a major achievement for peace and stability in Europe", the EC reported.
"The treaty will also create, firstly, an agreed policy framework for the World Bank and the EBRD support to infrastructure investments - which are estimated at USD30 billion in the electricity sector to reach EU standards by 2015 - and, secondly, the expansion of the natural gas system to create an intermediate gas market between the Caspian Sea and the European Union," the EC said in the statement,