The US moved its Ukrainian embassy operations from Kyiv to the western city of Lviv due to the threat of Russian invasion as Western leaders pushed for a diplomatic solution to the crisis. President Joe Biden and UK PM Boris Johnson agreed in a phone call on Monday night that there "remained a crucial window for diplomacy," Downing Street said in a statement. Western leaders have issued repeated warnings to Russia that any military aggression in Ukraine would result in serious consequences. The warnings come after Russia amassed more than 100,000 troops on Ukraine's border and made several security demands to the North Atlantic Treaty Alliance (NATO). Russia wants NATO to ensure it will not expand to Ukraine or any other former Soviet countries, to halt weapon deployments to Ukraine and roll back its forces in Eastern Europe. Ned Price, the US State Department spokesperson, said on Monday that there was a "continued and unprovoked Russian build-up on the border with Ukraine and no accompanying evidence of de-escalation." "It is a distinct possibility -- perhaps more real than ever before -- that Russia may decide to proceed with military action, with new Russian forces continuing to arrive at the Ukrainian border and Russian forces staged all around Ukraine, an invasion, as we have said, could begin at any time." Russia has denied plans to invade its neighbouring country and signalled that it was open to continued talks with NATO. Foreign Minister Sergei Lavrov told President Vladimir Putin on Monday that Moscow should hold more talks with the US and its allies despite their refusal to consider Russia's main security demands. US Secretary of State Anthony Blinken said on Monday that the US was "in the process of temporarily relocating our Embassy operations in Ukraine from our Embassy in Kyiv to Lviv due to the dramatic acceleration in the buildup of Russian forces." He emphasised that the "prudent precautions in no way undermine our support for or our commitment to Ukraine." Ukrainian President Volodymyr Zelenskyy had earlier characterised moving embassy operations to the western part of the country as "a big mistake." "It's a big mistake that some embassies moved to western Ukraine," Zelenskyy said at a press conference with German Chancellor Olaf Scholz. "It's their decision, but 'western Ukraine' doesn't exist, it's a united Ukraine. If something happens, God forbids, it (a military escalation) will be everywhere. It's not possible to be five or six hours away from escalation or troubles." UN Secretary-General Antonio Guterres said on Monday that he had spoken with foreign affairs ministers in Russia and Ukraine and that his message was clear. "There is no alternative to diplomacy. All issues, including the most intractable, can and must be addressed and resolved through diplomatic frameworks. It is my firm belief that this principle will prevail," he said. Germany's Scholz will travel to Moscow on Tuesday after visiting Kyiv in a show of support for Ukraine. Scholz reiterated his warning that "further military aggression against Ukraine would have serious political, economic and geostrategic consequences for Russia". He said on Monday that NATO and the US had made proposals to Moscow that Germany supports, "and we now expect a reaction, an answer to them from Russia." He urged Russia to accept offers of dialogue. There was no shift however in Germany's refusal to join some allies in supplying Ukraine with lethal weapons. Additional US F15 fighter jets landed in Poland on Monday, according to Mariusz Błaszczak, Poland's defence minister. Zelenskyy said the tensions around his country's future present "an unprecedented challenge for Europe and the world." "It is in Ukraine that the future of the European security architecture — of which our state is a part — is being decided today." — Euronews