By Robin Emmott and Sabine Siebold
BRUSSELS (Reuters) -U.S. Secretary of State Antony Blinken offered the United States full support to Ukraine in its conflict with Russia on Tuesday as NATO called on Moscow to halt a large troop build-up on Ukraine's borders.
Blinken, returning to Brussels just weeks after his maiden trip to Europe as U.S. President Joe Biden's top diplomat, told reporters that "the U.S. stands firmly behind the sovereignty and the territorial integrity of Ukraine". He said he would discuss Kyiv's ambitions to one day join NATO.
Blinken met with Ukrainian Foreign Minister Dmytro Kuleba in an intensification of diplomacy after the Group of Seven foreign ministers condemned the increase in Russian troop numbers, which the West says has gone unexplained.
Echoing NATO Secretary-General Jens Stoltenberg, who met Kuleba earlier, Blinken said Moscow was massing forces in its biggest build-up since 2014, when conflict flared between Ukraine and Russia, and Moscow annexed Crimea from Ukraine.
Blinken warned of Russia's "very provocative actions", while Stoltenberg said Russian soldiers were combat-ready.
"In recent weeks Russia has moved thousands of combat-ready troops to Ukraine's borders, the largest massing of Russian troops since the illegal annexation of Crimea in 2014," Stoltenberg said.
"Russia must end this military build-up in and around Ukraine, stop its provocations and de-escalate immediately," Stoltenberg said at his news conference with Kuleba.
Russia has said it moves its forces around as it sees fit, including for defensive purposes, and has regularly accused NATO of destabilising Europe with its troop reinforcements in the Baltics and Poland since the annexation of Ukraine's Crimea.
Russian news agencies quoted Defence Minister Sergei Shoigu as saying that Russia was taking measures to respond to what it calls threatening military action by NATO, including combat readiness checks and troop exercises.
Shoigu was cited as saying that NATO was planning to deploy 40,000 troops and 15,000 military personnel close to Russian territory. The Western alliance denies any such plans.
Russia on Tuesday also called the United States an "adversary" and told U.S. warships to stay away from Crimea, which is strategically-located on the Black Sea and is increasingly patrolled by NATO allies.