Belgrade/Sofia  - Gas shortages triggered by the Russia- Ukraine row over payments have left tens of thousands of families in Serbia and Bulgaria without the ability to heat their homes at a time of freezing cold.

While most Serbian plants have switched or were in the process of switching from natural gas to oil, some - as in the cities of Novi Sad and Pancevo, servicing some 120,000 people - have shut down as gas supplies were exhausted, reports said.

In Pancevo, the heart of Serbia's petrochemical industry just 15 kilometres east of Belgrade, an institution caring for more than 200 elderly people was in a critical situation, the daily Blic said.

Russia closed the valves on pipelines running through Ukraine, accusing Kiev of stealing the gas intended for users beyond its border.

The move left the entire Balkans without gas, with the situation aggravated by a spate of freezing weather. Serbia was hit particularly hard as it was virtually without any reserves.

After having to halt much of the gas-dependent industry, Serbs are nervously casting an eye on the electricity grid which has been overburdened as a result of the crisis.

Despite Belgrade authorities' repeated warnings that a further increase in electricity consumption could bring down the system, most people have no alternative to electric heaters that remain a staple household item supplementing Serbia's often unreliable central heating systems.

More than a third of Bulgaria's 7 million people are similarly dependant on remote heating systems and were suffering in the cold as heating plants were trying to switching from gas to oil.

A rush for electric heaters was reported in Bulgaria, along with increased concern for the electric power grid.

Bulgaria has had sufficient gas reserves for a week, but on Thursday ordered austerity measures, calling on the industry to shrink consumption from the normal winter usage of 12 million cubic metres to just 4.5 million.

Also cut off from the Russian-Ukraine gas supply in the Balkans were also Romania, Greece, Turkey, Macedonia, Bosnia, and Croatia as well as Albania and the former Serbian province Kosovo which normally have supply problems in winter.