Bulgaria remains one of the worst hit by the Russia-Ukraine gas row, as gas Russian supplies are suspended for a fourth day.
With temperatures as low as -17C and heavy snow covering most of the country, many schools remain closed and industry gas-rationing continues.
Reserves are currently estimated to be sufficient for 110 days.
Bulgaria receives all of its gas from Russia via Ukraine and has no access to any alternative gas routes or suppliers.
It is therefore among the hardest hit by the current block on deliveries triggered by a pricing and payments row between Moscow and Kiev.
A quarter of Bulgaria's 7.6m citizens, primarily in the big cities, rely on heating from gas-fired plants that have been forced to cut back supply and switch to oil.
The government promised today that heating would return to normal 'in the coming days,' but scores of schools across the country, including 28 in Sofia, remained shut.
Justice Minister Meglena Tacheva has expressed concern for prisoners, since all of the country's 11 prisons use gas for heating.
And unlike pupils in schools, 'inmates can't be sent home,' she said.
Gas monitors begin work in Kiev
EU observers tasked with monitoring the flow of Russian gas through Ukraine have begun work in Kiev, fulfilling the key condition for supplies to resume, the European Commission said.
'The team has begun its work at the central dispatching point of Ukrtransgas in Kiev,' the commission said in a statement.
'This means that all the conditions agreed between the leaders of the European Union, Russia and Ukraine are in place for the immediate restart of gas supplies from Russia that are destined for European customers,' it added.
The team consists of 20 members including experts from major gas companies and senior officials from the EU.
'The team of EU observers will be able to verify, on an independent basis, the data on the flow of gas deliveries to Ukraine and compare it with the data of gas volumes that will reach customers in the EU,' it said.
