BISHKEK, Kyrgyzstan : The United States on Tuesday shut its airforce base in Kyrgyzstan that had been the main transit hub for troops going to Afghanistan, a move expected to boost Russia's influence in Central Asia.Washington was forced to shut the Manas Transit Centre north of Bishkek after Kyrgyzstan, an ex-Soviet country seeking closer ties with Moscow, refused to extend its lease last year. US officials gave the Kyrgyz authorities a symbolic set of wooden keys at a ceremony to hand over the base, located just outside the capital Bishkek. US ambassador to Kyrgyzstan Pamela Spratlen said all military personnel would leave the base in a week.Some 5.5 million coalition troops passed through the base since it was established in late 2001, in the wake of the September 11 attacks in the US, commander John Millard said. The US rented the base in the country's main civilian airport for $60 million a year, and many locals have for years been irritated by foreign military presence.It has now moved its main transit base for Afghanistan to Romania as it gears up to withdraw its troops altogether to end America's longest war.Russia maintains its own airforce base in Kyrgyzstan as it vies for influence in Central Asia.Kyrgyzstan has for years used the Manas air base as a bargaining chip to get aid from both Russia and the United States.It pulled the plug on the US base as it seeks to join the Eurasian Economic Union, which launches in January and will include Russia and ex-Soviet Belarus and Kazakhstan."We didn't even have a choice," said Kyrgyz political analyst Chingiz Shamshiyev. "Russia is the partner that we inherited thanks to our history."But another political analyst, Mark Sariyev, said that the shutdown of the base did not mean that the United States was leaving the country for good."Possibly US bases could be established in the country's south - Kyrgyzstan could offer Americans such an option," said Sariyev. Asanbek Alymkozhoyev, chief of the general staff of Kyrgyzstan's armed forces, told reporters that the base would now host the country's National Guard.The authorities are now seeking investors to help turn the airport into a modern transportation and logistics hub. Both Russian and Chinese companies have expressed interest.Russian state oil giant Rosneft said in February it had signed a preliminary agreement that would allow it to buy at least 51 percent in the Manas airport.The company said it could spend up to $1 billion on the project.