UPDATE 3-Russia's Medvedev wades into Ukraine polls
* Medvedev says Ukraine putting gas flows to Europe at risk
* No new ambassador to Kiev for now
* Moves aimed at January presidential election
(Adds context and analysis)
MOSCOW, Aug 11 (Reuters) - Russian President Dmitry Medvedev waded into neighbouring Ukraine's presidential election campaign on Tuesday, attacking the incumbent as "anti-Russian" and urging the next leader to cooperate with Moscow.
In an open letter to Ukraine's pro-Western President Viktor Yushchenko, Medvedev said he would postpone sending a new ambassador to Kiev, and accused Yushchenko of putting gas supplies to Europe at risk by disrupting ties with Moscow.
Ukraine, a former Soviet republic whose population is divided between pro-Western and Russia-leaning camps, elects a new president in January to succeed Yushchenko, swept to power in the 2004 Orange Revolution.
Disputes over Russian gas supplies to Ukraine have led to two disruptions in supplies to Europe in the past three years and to Western accusations -- denied in Moscow -- that Russia is using gas as a political weapon.
"We have the impression that Kiev consistently seeks to break traditional economic ties with Russia, first and foremost in the energy sector," Medvedev said in the letter posted on his website (www.kremlin.ru).
"As a result, the stable use by our countries of what is effectively a single gas pipeline network serving the energy security of Russia, Ukraine and many European states has been put at risk," Medvedev wrote.
"... on account of the anti-Russian course of the Ukrainian leadership, I have decided to postpone sending our new ambassador to Ukraine."
There was no immediate reaction from Yushchenko but Ukraine's acting foreign minister, Volodymyr Khandohyi, said the letter "has reached the president and it is being studied".
Yushchenko's political rival Viktor Yanukovich, who heads the biggest party in parliament, said Moscow would not see any improvement in relations while Yushchenko was in power:
"The first thing we will do on taking power is to revive normal, good neighbourly, equal and mutually beneficial relations with our strategic partner Russia."
Analysts said Medvedev's blunt message was timed to influence the campaign for Ukraine's presidential vote on Jan. 17.
"This is a tough diplomatic demarche," said Vladimir Fisenko, head of the Ukrainian think-tank Penta. "This is a signal for the presidential campaign, aimed against Yushchenko."
"It is also a signal to all Ukrainian politicians that it's time for Kiev to change its course."
Yushchenko, whose approval rating stands around 4 percent, has little chance of winning re-election. Yanukovich, whom Russia backed in the last election, leads with up to 24 percent.
Prime Minister Yulia Tymoshenko stands at around 14 percent, and Arseny Yatsenyuk, a pro-Western former parliamentary speaker and foreign minister, at 10 percent. An alliance between Tymoshenko and Yatsenyuk could challenge Yanukovich.
In his letter, Medvedev reiterated Russian criticism of Yushchenko for trying to lead Ukraine into NATO despite polls indicating that most Ukrainians are opposed.
He also attacked Yushchenko for seeking to expel Russia from its Black Sea naval base in Sevastopol. "What we have seen throughout the years of your presidency cannot be viewed as anything other than a departure by the Ukrainian side from the principles of friendship and partnership with Russia," he said.
"Russia hopes a new Ukrainian leadership will be ready to build ties between our countries, ties that will indeed answer the true hopes of our peoples in the interests of strengthening European security."
Ukraine last month expelled a Russian diplomat for "actions incompatible with his status", prompting Russian retaliation.
Medvedev was also irritated by Ukraine's slowness in confirming his appointment of a new Russian ambassador -- whose move is now on hold.
Alexei Mukhin, the head of the independent Russian think-tank Centre for Political Technologies, said gas was the Kremlin's main source of concern.
"Russia is preparing for delays in gas payments in December and January," Mukhin said. "Many in Ukraine are likely to try and use the gas card in their election campaign."
Yushchenko has criticised a deal on gas prices and transit that Tymoshenko struck with Russian Prime Minister Vladimir Putin in January.
An EU-Ukraine deal on rebuilding Ukraine's gas network infuriated Moscow because Russia was not involved, and Medvedev mentioned this again in a posting on his website.
"The supreme Ukrainian leadership is negotiating the supplies of gas -- Russian gas -- with the European Union, bypassing Russia, and is signing a document contradicting the Russian-Ukrainian deals of January," he wrote. (Additional reporting by Yuri Kulikov and Pavel Polityuk in Kiev; Editing by Michael Stott and Kevin Liffey)