A trilateral summit took place in the Bulgarian city of Varna earlier in the day where the three leaders discussed energy security, among other topics.
"We agree to further cooperate in enhancing the Alliance's capacity to ensure adequate and sustainable fuel supply to NATO's Eastern Flank," the declaration, released by the Greek Prime Minister's Office, read.
The leaders of the three countries expressed support for a "continuous NATO focus and strong NATO presence in the Black Sea," which they see as key "for ensuring deterrence and prevention of conflicts in the Southeastern flank of the Alliance," according to the declaration.
Regarding the rail connections between Greece, Bulgaria and Romania, the countries' leaders agreed to develop new routes and corridors on the North-South axis. They also intend to further promote the Vertical Gas Corridor, the declaration said.
"This project, together with the Greece-Bulgaria Interconnector and FSRU [Floating Storage Regasification Unit] in Alexandroupolis, will contribute significantly to the diversification of supplies, the EU energy security and the energy independence of the wider region, including the Republic of Moldova and potentially Ukraine," the declaration added.
The prime ministers agreed to hold the next meeting in 2024, according to the statement.
NATO has been boosting its eastern flank along the borders with Russia and Belarus since late 2021 by dispatching additional troops. In response to Moscow launching a military operation in Ukraine in February 2022, NATO has further reinforced the flank by sending more ships, planes and ground troops to the region and putting them on high alert, as well as conducting regular military drills in the area.