MOSCOW - President Vladimir Putin warned Wednesday Russia may begin requiring advance payment for gas supplies unless Ukraine comes to the negotiating table over its unpaid energy bills.Russia’s state-controlled natural gas company “Gazprom will only send gas in the amounts that the Ukrainian side has paid for a month in advance” under the changed sale terms being contemplated, Putin said at a government meeting according to televised excerpts. “They will receive as much as they have paid for,” he said. Russia says Ukraine now owes it $2.2 billion for natural gas supplies, and Gazprom last week demanded that Ukraine “take immediate measures” to settle the debt.Prime Minister Dmitry Medvedev insisted during the meeting that the situation was “critical” and urged the government to switch to a system of advance payments.Putin however said that for now Russia would refrain from doing so, citing Ukraine’s economic difficulties and Moscow’s ongoing talks with the European Union.Ukraine now has to pay $485 dollars for 1,000 cubic metres of gas, the highest price of any of Gazprom’s clients in Europe.Speaking at the government meeting, Putin added that Russia was not in a position to prop up Ukraine’s struggling economy indefinitely.“The situation today is strange to say the least. As we know, our partners in Europe recognise the legitimacy of Kiev’s current authorities but are doing nothing to support Ukraine. Not a single dollar, not a single cent.“The Russian Federation does not recognise the legitimacy of the authorities in Kiev but is still continuing to render it economic assistance and subsidise Ukraine’s economy in the amount of hundreds of millions and billions of dollars,” Putin said. “Of course, this cannot last forever.”Putin also expressed hope that Kiev would not take any irreversible steps and Russia’s diplomatic efforts would be successful.“I hope that the acting (authorities) will not do anything that can’t be fixed later,” he said, expressing hope that Russian efforts to calm tensions through talks would have a “positive” result.Meanwhile, pro-Russian separatists reinforced barricades around the state security building in the eastern Ukrainian city of Luhansk on Wednesday and called on President Vladimir Putin for help after the government warned it could use force to restore order. But protesters were also engaged in talks to ease the standoff, which Kiev has said could provide a pretext for a Russian invasion, and lawmakers from eastern Ukraine proposed an amnesty for protesters to defuse tension.The former KGB headquarters is one of three government buildings seized this week in eastern Ukraine by protesters demanding regional referendums on independence from Kiev, like the one in Crimea that led to its annexation by Russia.Tensions have risen in the mainly Russian-speaking east since the overthrow of Ukraine’s Moscow-backed president and the installation of a new pro-European government.“Of course we must ask Russia to take us in because I don’t see an alternative,” said a man dressed in camouflage who gave his name as Vasiliy and said he was the commandant of the building. “Putin help us!” he said.Sandbags and wooden crates were piled near the entrance of the building to defend it against the police. Men with rifles could be seen through broken windows above.Local police spokeswoman Tatyana Pogukai said protesters had found an arsenal of weapons within the building. Protesters say they have 200-300 Kalashnikov automatic rifles. She denied previous reports that hostages had been taken.She said negotiations had been carried out overnight but the two sides had not come to an agreement.“They won’t put down their weapons until there is agreement on a referendum,” she said.Protesters in Donetsk, to the south, remain in control of the main regional authority building, but authorities have ended the occupation in the city of Kharkiv.“A resolution to this crisis will be found within the next 48 hours,” Interior Minister Arsen Avakov told reporters in the capital Kiev.“For those who want dialogue, we propose talks and a political solution. For the minority who want conflict they will get a forceful answer from the Ukrainian authorities,” he said.Ukraine’s state security service said that 50 people had left the building in Luhansk overnight. Protesters confirmed that some had left.Activists, many in balaclavas and masks, continued to build makeshift barricades and prepared petrol bombs.“Those who left were not ready to stay and fight,” said Vasiliy, who said his “soldiers” would fight on until a referendum on independence from Kiev was held.Ukraine’s government says the actions are part of a Russian-led plan to dismember the country, a charge Moscow denies.US Secretary of State John Kerry accused Russian agents and special forces on Tuesday of stirring up separatist unrest and said Moscow could be trying to prepare for military action as it had in Crimea.Russia denied the accusations on Wednesday and dismissed concerns over a troop buildup near the border with Ukraine in what has become the worst East-West crisis since the end of the Cold War in 1991.