MOSCOW, Oct 21 (Reuters) - President Vladimir Putin said on Thursday that the Western-backed military development of Ukraine posed a serious threat to Russia, two days after the U.S. defense secretary staged a show of support for Kyiv and encouraged its aspiration to join NATO. Putin told a group of journalists and Russia experts that Tuesday"s visit to Ukraine by Lloyd Austin, in which he said no third country had the right to veto its hoped-for NATO membership, had effectively paved the way for Kyiv to join. Whether it did or not, Putin said, Russia"s interests were targeted. "Formal membership (of Ukraine) in NATO may not take place, but military development of the territory is already under way," Putin told the Valdai Discussion Club. "And this really poses a threat to Russia. We are aware of that." The United States has been Ukraine"s most powerful backer since Russia"s 2014 annexation of Crimea and the outbreak of a war the same year between Russian separatists and government forces in eastern Ukraine, which Kyiv says has killed 14,000 people. The Kremlin chief has consistently made clear that NATO membership for Ukraine - which shares close ties with Russia going back to ancient times - would be a red line for Moscow. Russia this week effectively severed diplomatic relations with NATO after the alliance kicked out eight members of its mission there for alleged spying. NATO defence ministers agreed a new master plan on Thursday to defend against any potential Russian attack on multiple fronts. read more Adding to tensions with Washington, Russia has reported three instances in the past week when it says its forces intercepted a U.S. ship or aircraft on the point of breaching its waters or airspace, though the United States has disputed the details. read more In a wide-ranging discussion in southern Russia lasting several hours, Putin was more upbeat, however, on relations with the administration of President Joe Biden. He said talks between Moscow and Washington on strategic stability and cybersecurity were moving in the right direction, and that a summit between the two men in Geneva in June had been productive. Putin also said Biden had been right to withdraw U.S. troops from Afghanistan, whose new Taliban government sent representatives to Moscow this week for talks on rebuilding the country after the end of the 20-year U.S. troop presence. He said Afghanistan should get its financial assets unfrozen in the interest of boosting its stability - a position opposed by Washington, which has said it has no plans to release billions in Afghan gold, investments and foreign currency reserves parked in the United States and frozen after the Taliban seized power in August.