WASHINGTON, Oct 25 (Reuters) - U.S. President Joe Biden"s global energy security adviser said on Monday that Russian President Vladimir Putin is getting close to using natural gas as a political tool if Russia is holding back fuel exports to Europe as it suffers an energy crunch. "I think we are getting close to that line if Russia indeed has the gas to supply and it chooses not to, and it will only do so if Europe accedes to other demands that are completely unrelated," Amos Hochstein, Biden"s adviser, told reporters, when asked if Putin was using gas as a weapon. Hochstein said gas prices in Europe have been driven higher not just by events in the region but also by a dry season in China that has reduced energy output from hydropower and increased global competition for natural gas. Still, while several factors have led to the European gas crisis, Russia is best placed to come to the aid of Europe, he said."There is no doubt in my mind, and the (International Energy Agency) has itself validated, that the only supplier that can really make a big difference for European energy security at the moment for this winter is Russia," Hochstein said. Russia can increase upstream production of gas, and should do it quickly through existing pipelines, he said. Putin has rejected suggestions that Moscow was squeezing supplies for political motives, saying it will increase flow as much as partners ask. read more Putin has blamed record high prices on the EU"s energy policy and said Russia can boost supplies to Europe once its Nord Stream 2 gas pipeline gets approved. read more Yuriy Vitrenko, the head of Ukraine"s state energy company Naftogaz, this month said Russia was trying to blackmail Europe into certifying its Nord Stream 2 gas pipeline by keeping fuel supplies low. The pipeline, which Washington opposes because it would circumvent Ukraine, is finished but needs approvals from Germany to start delivering Russian gas under the Baltic Sea to Europe. Approvals from Germany and the European Commission for Nord Stream 2 will likely take until March, so if Russia says it can quickly boost gas flow through Nord Stream 2, it should be able to do so now through existing pipelines, Hochstein said. "You can"t have it both ways," Hochstein said