Italy has had to dip into its gas reserves to make up for a shortfall in supplies from Russia, Italian energy group ENI has said.
The EU is growing worried about its dependence on Russian gas
It says it had to use four million cubic metres of gas from reserves because supplies were 5% below demand.
Russia has denied reports that it is holding back gas for its domestic market because of freezing conditions.
Italy's Industry Minister Claudio Scajola has called a crisis meeting with energy firms to discuss the issue.
The announcement by ENI came after news agencies reported that Russia's state-owned Gazprom had cut supplies by 20% to parts of Europe because of cold weather.
According to the reports, gas flows had been cut on the route to Hungary, which also supplies Bosnia-Herzegovina.
But Gazprom spokeswoman Olga Moreva, quoted by AFP news agency, described the reports as "incorrect".
"We are fulfilling all our obligations," she said.
Earlier on Wednesday, Austrian Chancellor Wolfgang Schuessel called on the EU to reduce its dependence on Russian gas supplies.
Mr Schuessel said the EU had to ensure "security of supply" after a gas row between Russia and Ukraine caused temporary shortages in Europe.
The Austrian leader was setting out his country's priorities for its six-month presidency of the EU.
He told the European Parliament in Strasbourg that the EU needed to promote long-term investment in energy and "look for a long-term solution".
The EU imports more than 40% of the gas it uses. About half of that comes from Russia via pipeline through Ukraine.
Russia is renegotiating gas deals with a number of ex-Soviet Union nations, a move that has raised regional tensions.
On 1 January it cut supplies to Ukraine, raising the prospect of surging gas prices across Europe.
The spat with Ukraine was short-lived, but caused a 19% decline in gas deliveries to Italy.