By Richard Balmforth
KIEV (Reuters) - European Union mediators resumed a push on Tuesday to secure the release of jailed Ukrainian opposition leader Yulia Tymoshenko, with EU politicians warning time was running out before the signing of a landmark trade agreement next month.
Russia, dismayed by the westward course of its former Soviet ally, gave vent to its anger again, charging Ukraine with being way behind in its payments for gas deliveries. Its words revived concerns of a new gas "war".
The agreements on association and free trade, to be signed at an EU-Ukraine summit on November 28, offer the former Soviet republic of 46 million people the chance of a historic shift west away from Russia.
But signature hinges on the release of ex-prime minister Tymoshenko, a fierce opponent of President Viktor Yanukovich. She was jailed in 2011 for seven years for abuse of office after a trial which the EU says was political.
Her case has become symbolic for the 28-member EU of what it calls "selective justice" in Ukraine, which it wants ended before the summit in Vilnius, Lithuania.
Last week, EU ministers warned Yanukovich the clock was against him for reaching a deal that could ensure the signing.
Irish politician Pat Cox and former Polish President Aleksander Kwasniewski, who have shuttled in and out of Ukraine for more than a year and a half in an effort to nail down a deal, arrived in Kiev on Tuesday and met Andriy Klyuyev, a close aide of Yanukovich.
A statement from Klyuyev's office said only they had discussed preparations for the signing in Vilnius.
The envoys are focusing attention on a compromise under which Tymoshenko, 52, can travel to EU member Germany to receive medical treatment for spinal problems.
Yanukovich has balked at granting her a pardon but says he will sign into law any draft from parliament which would allow her to take a break from prison and go abroad for treatment.
Her supporters are still pressing for a "full amnesty" so she can be cleared of her jail sentence and free to return to politics one day. She said on Friday however she would accept any compromise agreeable to the two-man mediation mission.
"We are determined to do everything we can to encourage the Ukrainian authorities to find the solution acceptable to all sides in Ukraine and acceptable to any host state in the European Union that would receive Mrs Tymoshenko for treatment if she's released on humanitarian grounds," Cox said in Lithuania on Monday.
"This remains the core objective of our mission, and we are determined to do everything we can to make it succeed."
EU foreign ministers are due to give an assessment of Ukraine's record at a meeting on November 18 and decide whether it has met the key criteria, including ending "selective justice", to allow the signing to go ahead.
The EU is split between states, such as Poland, which stress the need to prise Ukraine away from Russia's embrace and those, like Sweden and the Netherlands, which insist the bloc not compromise principles of civil rights and justice.
Cox and Kwasniewski may meet Yanukovich himself during their stay. They were also due to go to the northern town of Kharkiv to see Tymoshenko, who is in hospital under prison guard there.
As the two envoys began their program, there was a new show of anger from Russia, which has tried unsuccessfully to persuade Ukraine to stay in the ex-Soviet family and join a Moscow-led Customs Union with other ex-Soviet republics.
The Russian gas export monopoly Gazprom demanded Ukraine's Naftogaz pay an overdue gas bill "urgently", saying it was extremely concerned about the debt.
Cash-strapped Ukraine, which faces huge foreign debt repayments next year, gets nearly all its gas from Russia. Rows over pricing caused two gas wars in the winters of 2006 and 2008, with Moscow halting deliveries not only to Ukraine but also to the rest of Europe.
The tension over gas prices goes back to Tymoshenko's time as prime minister. The abuse of office charge for which she was sentenced relates to a 2009 deal with Russia which she brokered.
Yanukovich's government says that deal saddled the economy with an exorbitant price for gas - currently around $400 per thousand cubic meters - but Russia has refused to yield to the Kiev government's repeated entreaties to bring the price down.
(Writing by Richard Balmforth; editing by Andrew Roche)