Biden promised Zelensky that the US and its allies "will respond decisively" if Russia invades Ukraine. The two leaders held a telephone conversation this Sunday. The American expressed his support for the European president amid tensions with Moscow. On Sunday, US President Joe Biden told his Ukrainian counterpart, Volodimir Zelenski, that Washington and its allies "will respond decisively" if Russia acts to invade its pro-Western neighbor, the White House reported. Reporting on the phone chat between the two, White House press secretary Jen Psaki also said that Biden expressed support for the diplomatic efforts, including high-level talks between Russian and American officials scheduled for January 9-10. in Geneva. " President Biden made it clear that the United States and its allies and partners will respond decisively if Russia does invade Ukraine," Psaki said in a statement. Biden also "reaffirmed the United States' commitment to the sovereignty and territorial integrity of Ukraine," he added. The show of US support for Ukraine comes days after Biden warned Russian President Vladimir Putin of the dire consequences if Moscow invades its neighbor. In his call with Zelensky, Biden also emphasized Washington's commitment "to the principle of 'nothing about you without you,'" in an apparent reference to the need to include Ukraine in negotiations about its future. Zelensky later tweeted that he appreciated America's "unwavering support" and that the so-called "proves the special nature" of the two countries relationship. Earlier this week, Biden spoke to Putin, the two leaders' second phone conversation in three weeks as tensions over Ukraine mounted. Speaking about the call with Putin on Friday, Biden said: " I am not going to negotiate here in public, but we make it clear that you cannot, I will emphasize, that you cannot invade Ukraine ." In remarks to journalists during a vacation stay in Delaware, the US leader added that he had "made it clear to President Putin that we will have severe sanctions, we will increase our presence in Europe, with NATO allies" if Russia invades Ukraine. Biden also "expressed support for confidence-building measures to reduce tensions in Donbas and active diplomacy to advance the implementation of the Minsk Accords." Under the Minsk agreement, negotiated by France and Germany, Ukraine agreed to carry out political reforms. In contrast, Russia decided to end its support for pro-Russian separatist rebels. Washington and its European allies accuse Russia of threatening Ukraine with a new invasion. Some 100,000 Russian troops are concentrated near the country's border. Putin already seized the Crimea region in 2014 and is accused of inciting a pro-Russian separatist war that broke out that same year in the east. Moscow describes the presence of troops as protection against NATO expansion, even though Ukraine has not been invited to become a member of the military alliance. On Friday, US Secretary of State Antony Blinken spoke with NATO Chief Jens Stoltenberg; Blinken subsequently urged Russia to "engage significantly" in upcoming talks on the tense standoff between Moscow and Kyiv. Stoltenberg said that NATO was "united" and "ready for dialogue. " In Thursday's call, Biden warned Putin against invading Ukraine. At the same time, the Kremlin leader said sanctions against Moscow would be a "colossal mistake." After a 50-minute phone call, the second in just three weeks, both presidents indicated their support for more diplomacy. Putin was generally "satisfied" with foreign policy adviser Yury Ushakov told reporters about the talks. A senior US official, who spoke on condition of anonymity, said the tone "was serious and significant." However, the depth of the disagreement, or the dangerously great dangers on the margins of Eastern Europe, could not be hidden.