LAHTI, Finland (Reuters) -- Russia and Ukraine will sign an agreement within days to guarantee natural gas supplies for European customers this winter, European Union foreign policy chief Javier Solana said on Friday.

In January, a row between Russia and its ex-Soviet neighbor over gas prices briefly disrupted supplies to European countries because most of the Russian fuel on which Europe depends is shipped through pipelines that cross Ukraine.

"I can tell you this week an agreement will be signed between Ukraine and Russia and they will have a guaranteed supply of gas for all countries, so that what happened last year and the year before will not happen again," Solana told a news conference at an EU summit in Finland.

The EU foreign policy chief was in Ukraine on Thursday for talks and said he was briefed on the impending deal.

Russia's gas export monopoly Gazprom supplies about a quarter of Europe's gas needs and up to 80 percent of those volumes pass through Ukraine.

Ukraine's Prime Minister Viktor Yanukovich, on good terms with Moscow, has been trying to thrash out a deal under which Russia would increase supplies so that Ukrainian gas reservoirs are full before the winter, ironing out any possible disruption.

European supplies were briefly disrupted in January because Ukraine -- whose domestic supplies Russia had cut -- dipped into gas intended for Europe to meet domestic demand.

Yanukovich, who came to office after the gas row, still has to agree a long-term gas supply contract with Russia. Gazprom supplies gas to Ukraine at $95 per 1,000 cubic meters but that contract expires on Jan. 1. The Russian giant says it wants to raise the price, possibly as high as $230.