Bulgaria can ensure natural gas supplies for at least three to four months if a price dispute between Moscow and Kiev leads to supply disruptions, the country's government said.
Bulgaria is one of the European Union countries most exposed to possible gas supply cuts, as it gets almost all of its gas from Russia's Gazprom via one route through Ukraine.
Around 15 percent of EU's gas is supplied through Ukraine.
Russia cut off gas to Ukraine on Monday in a dispute over unpaid bills that could disrupt supplies to the rest of Europe and set back hopes for peace in the former Soviet republic.
More top news
Stephen Hawking said a Brexit would make Britain more "isolated and insular".
Michael Gove and Boris Johnson are increasingly behaving like an opposition party at a general election.
At least 17 people trapped inside an ammunition depot in India when a fire broke out are feared to have died.