MOSCOW (AP) -- Russian President Vladimir Putin and U.S. President Joe Biden are to hold a high-stakes telephone call on Saturday as tensions over a possibly imminent invasion of Ukraine escalated sharply and the U.S. announced plans to evacuate its embassy in the Ukrainian capital.
Before talking to Biden, Putin is to have a call with French President Emmanuel Macron, who met with him in Moscow earlier in the week to try to resolve the crisis.
Russia has massed well over 100,000 troops near the Ukraine border and has sent troops to exercises in neighboring Belarus, but insistently denies that it intends to launch an offensive against Ukraine.
However, Russian Foreign Ministry spokeswoman Maria Zakharova said Saturday that the country has "optimized" staffing at its embassy in Kyiv, but said the move was in response to concerns about possible military actions from the Ukrainian side.
"We conclude that our American and British colleagues apparently know about some military actions being prepared in Ukraine that could significantly complicate the situation in the security sphere," she said. "In this situation, fearing possible provocations by the Kyiv regime or third countries, we actually decided to somewhat optimize the staffing of Russian foreign missions in Ukraine."
Russian Foreign Minister Sergey Lavrov and U.S. Secretary of State Antony Blinken spoke by telephone Saturday. Lavrov told Blinken that "the propaganda campaign launched by the United States and its allies about 'Russian aggression' against Ukraine pursues provocative goals."
Britain on Saturday told its citizens to leave Ukraine. Armed Forces Minister James Heappey told the BBC that U.K. troops that have been training the Ukrainian army also would leave the country. Germany and the Netherlands also called on their citizens to leave as soon as possible.
Adding to the sense of crisis, the Pentagon ordered an additional 3,000 U.S. troops to Poland to reassure allies.
Biden has said the U.S. military will not enter a war in Ukraine, but he has promised severe economic sanctions against Moscow, in concert with international allies.
The timing of any possible Russian military action remains a key question.
The U.S. picked up intelligence that Russia is looking at Wednesday as a target date, according to a U.S. official familiar with the findings. The official, who was not authorized to speak publicly and did so only on condition of anonymity, would not say how definitive the intelligence was. The White House publicly underscored that the U.S. does not know with certainty whether Putin is committed to invasion.
However, U.S. officials said anew that Russia's buildup of offensive air, land and sea firepower near Ukraine has reached the point where it could invade on short notice.
A State Department travel advisory on Saturday said most American staff at the Kyiv embassy have been ordered to leave and other U.S. citizens should depart the country as well.
The Ukrainian Foreign Ministry on Saturday said "it is critically important to remain calm, to consolidate within the country, and to avoid actions that undermine stability and sow panic." It added that the armed forces "are constantly monitoring developments and are ready to rebuff any infringement on the territorial integrity and sovereignty of Ukraine."
Biden's national security adviser, Jake Sullivan, said Americans should not expect the U.S. military to rescue them in the event that air and rail transportation is severed after a Russian invasion.
Several NATO allies including Britain, Canada, Norway and Denmark also are asking their citizens to leave Ukraine, as is non-NATO ally New Zealand.
Sullivan said Russian military action could start with missile and air attacks, followed by a ground offensive.
"Yes, it is an urgent message because we are in an urgent situation," he told reporters at the White House.
"Russia has all the forces it needs to conduct a major military action," Sullivan said, adding, "Russia could choose, in very short order, to commence a major military action against Ukraine." He said the scale of such an invasion could range from a limited incursion to a strike on Kyiv, the capital.
Russia scoffed at the U.S. talk of urgency.
"The hysteria of the White House is more indicative than ever," said Russian spokeswoman Zakharova. "The Anglo-Saxons need a war. At any cost. Provocations, misinformation and threats are a favorite method of solving their own problems."
In addition to the more than 100,000 ground troops that U.S. officials say Russia has assembled along Ukraine's eastern and southern borders, the Russians have deployed missile, air, naval and special operations forces, as well as supplies to sustain a war. This week Russia moved six amphibious assault ships into the Black Sea, augmenting its capability to land marines on the coast.
Sullivan's stark warning accelerated the projected timeframe for a potential invasion, which many analysts had believed was unlikely until after the Winter Olympics in China end on Feb. 20. Sullivan said the combination of a further Russian troop buildup on Ukraine's borders and unspecified intelligence indicators have prompted the administration to warn that war could begin any time.
"We can't pinpoint the day at this point, and we can't pinpoint the hour, but that is a very, very distinct possibility," Sullivan said.
Biden has said U.S. troops will not enter Ukraine to contest any Russian invasion, but he has bolstered the U.S. military presence in Europe as reassurance to allies on NATO's eastern flank. On Friday the Pentagon said Biden ordered a further 3,000 soldiers to Poland, on top of 1,700 who are on their way there. The U.S. Army also is shifting 1,000 soldiers from Germany to Romania, which like Poland shares a border with Ukraine.
Biden spoke to a number of European leaders on Friday to underscore the concerns raised by U.S. intelligence about the potential imminence of a Russian invasion.
Russia is demanding that the West keep Ukraine and other former Soviet countries out of NATO. It also wants NATO to refrain from deploying weapons near its border and to roll back alliance forces from Eastern Europe -- demands flatly rejected by the West.
Russia and Ukraine have been locked in a bitter conflict since 2014, when Ukraine's Kremlin-friendly leader was driven from office by a popular uprising. Moscow responded by annexing the Crimean Peninsula and then backing a separatist insurgency in eastern Ukraine, where fighting has killed over 14,000 people.
A 2015 peace deal brokered by France and Germany helped halt large-scale battles, but regular skirmishes have continued, and efforts to reach a political settlement have stalled.