Greece, Bulgaria, Romania and Hungary have agreed to upgrade the interconnection and transmission capacity of their gas networks as part of their long-standing efforts to diversify gas sources and increase their role in Europe's energy supply chain, Reuters reported.

In 2016, the four countries agreed to develop the necessary infrastructure to implement the so-called Vertical Gas Corridor, which will provide bidirectional gas flows from Greece to Northern Europe via Bulgaria, Romania and Hungary.

According to a memorandum of understanding signed on the margins of the liquefied natural gas (LNG) conference in Athens, Greece's DESFA, Bulgaria's Bulgartransgaz, Hungary's FGSZ and Romania's SNTGN Transgaz have agreed to start negotiations on the regulatory and economic aspects of the corridor.

The agreement is for three years with the possibility of one more extension, the company added.

"(It) is an important step towards the implementation of the Vertical Corridor, a project that will significantly contribute to the supply security of the wider region," Chief Executive of DESFA Maria Rita Galli said in the statement. She added that it will further strengthen the role of Greece and the entire region in the energy sector.

ICGB, owner of the second gas pipeline between Greece and Bulgaria (IGB) - a key part of the Vertical Gas Corridor that started operating this year - and Gastrade, developer of the floating storage and regasification facility off the northern Greek port of Alexandroupolis, also signed an agreement.

IGB will become more important when its maximum annual capacity increases to 3 billion cubic meters.

Greece began exporting gas to Bulgaria, whose supplies from Russia were halted earlier this year after it refused to pay in Russian rubles.

The latest development fits well with Greece's plans to become the main transit route for liquefied gas in Europe.

This year, Athens has more than halved its imports of Russian gas, instead increasing LNG supplies, mostly from the United States and Egypt, to its only LNG plant in Revitus near Athens.