U.S. President Joe Biden told Russia’s Vladimir Putin on Saturday that the West would respond decisively to any invasion of Ukraine, adding such a step would produce widespread suffering and isolate Moscow.

In the latest effort to manage growing hostility, the two men spoke by phone for an hour a day after Washington and its allies warned that the Russian military, which has 100,000 troops massed near Ukraine, could invade at any moment. Moscow has dismissed those warnings as “hysteria.”