By Dmitry Zhdannikov

MOSCOW, Feb 12 (Reuters) - Ukrainian President Viktor Yushchenko flies to Moscow on Tuesday to try to settle a row over debts for gas that threatens to cut Russian supplies of the fuel to Ukraine and put stable transit flows to Europe at risk.

Russia's gas export monopoly Gazprom (GAZP.MM: Quote, Profile, Research) has extended to Tuesday evening its deadline for cutting a quarter of gas supplies to Ukraine after last-ditch talks between companies from the two countries failed to resolve the dispute.

Russia supplies a quarter of Europe's gas, most of it via Ukraine, and European customers are hoping the row will not escalate into a repeat of early 2006, when a pricing dispute between Moscow and Kiev disrupted shipments to Europe.

Although Gazprom pushed back the deadline to 1500 GMT on Tuesday from 0700 GMT, to allow Yushchenko to see Russian President Vladimir Putin first, analysts said the meeting of the two leaders did not guarantee the dispute would be settled.

"The situation with Ukraine looks very difficult especially because Ukraine lacks a united position on the scheme of future supplies. We consider chances of Gazprom going ahead with the threat and reducing deliveries as very high," said Alexander Razuvayev, head of market analysis at Russian bank Sobinbank.

Both Ukraine and Russia have assured Europe that westward gas flows will not be interrupted.

Gazprom says Ukraine owes it $1.5 billion in debts for earlier supplies.

Ukraine's pro-Western Prime Minister Yulia Tymoshenko, who clashed with Russia over gas during her earlier stint in office, has blamed Russian-imposed gas traders for racking up the debt and said Kiev could not pay it in full right away.