MOSCOW (REUTERS) - Russia agreed a US$15 billion (S$18.9 billion) bailout for Ukraine and slashed the price of gas exports on Tuesday under a deal that keeps the cash-strapped country in Moscow's orbit but fuelled street protests in Kiev.

Mr Vladimir Putin's lifeline to Ukrainian President Viktor Yanukovich was a triumph for the Russian leader in a geopolitical battle with the Europe Union. But the deal saddles Russia with a heavy financial burden and he failed to lure Ukraine into a customs union with other ex-Soviet republics.

Tens of thousands of protesters gathered in Kiev within hours of the agreement and accused Mr Yanukovich of selling his country to the highest bidder after walking away from a trade deal with the EU.

"He has given up Ukraine's national interests, given up independence and prospects for a better life for every Ukrainian," Mr Vitaly Klitschko, a protest leader and heavyweight boxing champion, told crowds on Kiev's Independence Square.

The United States said the deal would not address the concerns of the protesters, and German Chancellor Angela Merkel said Kiev should not be forced into allying itself with Moscow or the EU, to the exclusion of the other.

"At the moment it seems to be an either-or proposition. ...We need to put an end to this. Ukraine can't do this alone. Europe and Germany must continue to talk with Russia," she told ARD TV. "A bidding competition won't solve the problem."

The leaders of Ukraine and Russia clinched the deal at talks in the Kremlin that appeared to begin frostily but ended with them rubbing shoulders and laughing at a ceremony where documents were signed on reducing trade barriers for Ukraine.

Russian Finance Minister Anton Siluanov said Moscow would tap the National Welfare Fund, a rainy day fund, to buy US$15 billion worth of Ukrainian Eurobonds. The deal boosted the price of Ukraine's dollar debt, a sign of investors' confidence.

Underlining the urgency of Kiev's problems, Interfax news agency quoted Siluanov as saying Russia may buy US$3 billion in two-year Ukrainian bonds as soon as the end of this week.

Moscow also offered relief on the gas price. Ukraine's Naftogaz energy company will pay Russia's Gazprom US$268.5 per 1,000 cubic metres of natural gas, on which it is heavily dependent. The previous price had been about US$400.

"Ukraine is our strategic partner and ally in every sense of the word," Mr Putin said after the talks, with Mr Yanukovich sitting beside him in a gilded Kremlin hall.

"This (assistance) is not tied to any conditions," he added."I want to calm you down - we have not discussed the issue of Ukraine's accession to the customs union at all."

INVESTORS ENCOURAGED BUT SEE RISKS

Ukraine had been seeking help to cover an external funding gap of US$17 billion next year - almost the level of the central bank's depleted currency reserves.

Investors said the deal would stave off the immediate threat of default or a currency crisis but said there were also risks for Russia, whose own economy is stuttering.

"This is a rescue. Without that money, Ukraine would have defaulted sometime before the middle of next year ... And the cheap gas will provide a significant stimulus," said Mr Chris Weafer, senior partner with consultancy Macro-Advisory.

"The next move is for the protesters in Kiev." Ukraine, which had fears fuel supplies could be hit during the financial crisis, is caught between Western powers, keen to anchor the country in a friendly embrace on the EU's borders, and its former Soviet masters in Moscow.

Mr Yanukovich has been seeking the best possible deal for his country of 46 million but faces calls to resign at home and has been criticised in the West after police used force against the protests in the heart of Kiev.

The deal appears to preclude Ukraine looking West in the near future, though its leaders say they still see building ties with the EU as a possible long-term goal.

Commentators saw the bailout as Ukraine's reward from Moscow for scrapping the planned pact with Europe last month. "This refusal had a cost, and Russia has paid," Russian former economy minister Andrei Nechayev told Ekho Moskvy radio.

Moscow now has a financial hold over Ukraine: If it withdraws its money and alters the gas price, it could pull the plug on its neighbour. Mr Putin appeared to underline this by saying the agreements on the gas price and US$15 billion investment were temporary.

Russia also agreed to resume oil supplies to a refinery in Ukraine following a three-year break, traders said.

But Mr Putin will be disappointed if he cannot bring Ukraine into the Eurasian Union he plans to build with Kazakhstan, Belarus and other former Soviet republics to match the economic might of the United States and China.

Ukraine is by far the most populous ex-Soviet republic after Russia, and with its large market, mineral resources and borders with the EU, is vital to that project. Mr Yanukovich may be withholding Ukraine's membership to seek more concessions.

People at anti-government demonstrations in Kiev that have at times attracted hundreds of thousands fear Ukraine will now be stuck in Moscow's orbit, more than two decades after the fall of Soviet communist rule.

"With what has been signed now in Moscow, we can forget about Europe. Yanukovich made a massive mistake. He'd better not come back here, he'd better stay in Moscow," said Mr Deni Deyak, a businessman at the pro-Europe protest in Kiev.