Hundreds of thousands of homes in Europe remain without heating amid plunging temperatures as a gas row between Russia and Ukraine continues.
More than 15 countries have been hit by the shutdown of Russian supplies.
Serbia and Bosnia-Hercegovina are among the worst hit as many homes rely on heating stations that only run on gas.
The EU said it had reached agreement on an observer mission to monitor gas flows, but it was unclear when supplies would resume.
EU energy spokesman Ferran Tarradellas told the BBC that EU observers would leave for Ukraine on Friday.
But there has been no confirmation of the deal from the Russian side, which had, at the last minute, demanded Russian monitors be included on the team. See map of affected area
Ukraine's Naftogaz said it had no objection to Russians being included.
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It is hoped that once an agreement is set and the monitors start work, gas will start flowing back into Europe, the BBC's Gabriel Gatehouse in Kiev says.
But the acrimonious disagreement over gas prices between Ukraine and Russia remains, which leaves the crisis far from settled, he adds.
Protests
An estimated 100,000 people in Serbia were left without heating when Russian gas supplies to Europe were halted on Wednesday, the BBC's Helen Fawkes in Belgrade said.
With sub-zero temperatures across the country, at least eight towns and cities were completely cut off.
Dependence on Russia for gas:
Most of its gas-powered heating stations have switched to alternative energy but some can only operate on gas and have had to shut down completely, our correspondent adds.
It has also received emergency gas from Hungary and Germany.
In neighbouring Bosnia, some 72,000 homes were without heating in temperatures as low as -15C. Leaders warned its gas reserves would last only a few more days.
Angry Bulgarians protested in front of the Ukrainian embassy in Sofia on Thursday, holding placards accusing Russia and Ukraine of being "gas terrorists", the Associated Press reports.
Other countries reporting a total halt in gas supplies included Romania, the Czech Republic, Slovakia, Croatia, Greece, Hungary, Macedonia and Austria.
The Czech EU presidency said late Thursday, following talks with Russian and Ukrainian officials in Brussels, that agreement had been reached with Russia over the monitors.
"This deployment should lead to the Russian supplies of gas to EU member states being restored," it said in a statement, without specifying when monitors would be in place or when gas supplies would resume.
The EU depends on Russia for about a quarter of its total gas supplies, some 80% of which are pumped via Ukraine.
Russia cut gas to Ukraine itself a week ago as the row over pricing and allegedly unpaid bills escalated.
Ukraine denies Russian accusations that it is stealing gas passing through export pipelines on its territory.
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