European energy security depends in part on "realistic and feasible" infrastructure through central and southern European countries, a commissioner said Friday.

The European Commission announced the signing of a memorandum of understanding in Croatia with central and southern European countries that calls in part for more momentum behind the construction of what the commission said were "missing" gas links in the region.

Lingering economic and geopolitical issues in Ukraine pose a threat to European natural gas supplies. Europe gets about a quarter of its gas needs met by Russia's Gazprom, accused of holding monopolies in the region, and the majority of that volume runs through Soviet-era pipelines in Ukraine.

For Gazprom, its twin Nord Stream gas pipeline through the Baltic Sea and the planned Turkish Stream project, a scaled down version of the South Stream pipeline, offer means for diversification.

"The improvement of infrastructure through realistic and feasible projects is crucial to diversify energy resources and strengthen the region's resilience to supply shocks," European Energy Commissioner Maros Sefcovic said in a statement. "Cooperation among the countries of the region is key in this regard."

Bulgaria's decision in August to back away from South Stream led in part to a derailment of the project. The country's prime minister told European leaders in January that Bulgaria could become a common gas distribution center for EU member states in the region.

Gas links through Bulgaria, and pipelines stemming from Azerbaijan, are among the projects included in the European energy diversification strategy.

British energy company BP leads a project consortium tapping into natural gas off the coast of Azerbaijan. The company has awarded more than $1 billion in development contracts since selecting the Trans-Adriatic pipeline as its option for Azeri gas in 2013.

The European Commission said that, "ultimately, each member state in the region should have access to at least three different sources of gas."