Gazprom, Russia's state-owned natural gas monopoly, said it cut gas supplies to Ukraine by a quarter after Kiev refused to sign a new contract under which it would have to pay four times as much for the fuel.
"This is not a subject for discussion," President Yushchenko told a reporter, according to CNN.
"It is unacceptable not because the price is so high but because it is unfounded economically."
"I will call on [Russian President] Vladimir Putin... for Gazprom to return to the negotiating table," he said, as his country of 48 million people braced for shortages, with fears the dispute could affect deliveries to Europe, much of which is suffering a cold snap.
The dispute comes as Russia takes over as chairman of the Group of Eight.
Mr Yushchenko said Kiev had "never abandoned the negotiations" and Moscow is putting "economic pressure" on Ukraine.
He said hew would call his Russian counterpart to seek an end to the standoff.
Ukraine depends on Russia for about a third of its natural gas import needs.
All the gas now being pumped to Ukraine is usually shipped on to Europe, which gets 25 per cent of its gas from Russia.
Gazprom said gas deliveries to western Europe will not be disrupted, unless Ukraine covers its own shortfall by siphoning off transit supplies being piped westward across its territory.
But Ukraine's Naftogaz energy company has accused Russia of playing a dangerous game of brinkmanship that put Europe's gas supplies in jeopardy.
"Gazprom ... has cut supplies via a number of directions from which gas transit to Europe is carried out," Naftogaz said.
"The volumes of gas aimed at securing transit to Europe have been reduced ... Naftogaz declares such actions unacceptable because they endanger gas deliveries to Europe."
Ukraine's foreign ministry earlier accused Moscow of seeking to destabilise the country's economy.
The European Commission said it is concerned by the worsening crisis, but is confident the two neighbours can reach an agreement.
The United States also expressed regret over Russia's actions, saying it has created insecurity in the regional energy sector.
Supplies of Russian natural gas via Ukraine to Austria dropped by 18 percent on Sunday, Austria's OMV energy company said.
Austria draws 59 percent of its natural gas from Russia. Poland and Slovakia have also reported a drop in deliveries of Russian gas.
Ukraine's Western-leaning president has troubled Moscow by trying to take his ex-Soviet state on Russia's western border into NATO and the European Union.
Ukrainian officials say that is why the Kremlin is punishing Ukraine with such a huge price increase, while letting more Moscow-friendly ex-Soviet states such as Belarus go on paying far less for Russian gas.
Russia G8
Moscow has now taken over the annual chairmanship of the G8 club of industrialised nations for the first time from Britain, and its tenure is certain to come under intense international scrutiny.
"Russia wants to make energy security its key message to the G8 community, and simultaneously it is becoming a source of danger," said Valery Nesterov, energy analyst at Troika Dialog brokerage in Moscow.
Gazprom spokesman Sergei Kupriyanov said exports to Ukraine have been cut by 120 million cubic metres a day -- equivalent to Ukraine's total daily imports.
Eighty percent of Russian gas exports to western Europe pass through Ukraine.
Gas flow falling
Russia's NTV television, owned by Gazprom, quoted Alexander
Nemudrov, a Gazprom official at a pumping station in Slovakia, as saying gas flow out of Ukraine is already falling.
That suggests Ukraine is making up for its shortfall by drawing gas intended for other countries.
Gazprom officials in Moscow said they will not know definitely if that is the case until later on Monday.
The biggest European importers of Russian gas are Germany, Italy and France, which will have to draw down reserves of gas stored underground or seek alternative supplies if there is a major supply disruption.
Moscow is seeking a rise in the price of gas it sells to Ukraine at A$315 per 1,000 cubic metres from the current $A68, a level that reflects Soviet-era subsidised rates.
Ukraine agrees in principle but wants a transitional period.
Viktor Yushchenko, who propelled to power in the "Orange Revolution" a year ago, has linked the gas switch-off to the start of campaigning for a parliamentary election on March 26 in which he faces a tough challenge from pro-Moscow parties.