Update: 4:14 PM EST:
Reuters reports that Russia has vowed its own reprisals if western nations, including the U.S. and EU members, follow through with threats of sanctions against it:
"We will have to respond," Foreign Ministry spokesman Alexander Lukashevich said in a statement. "As always in such situations, provoked by rash and irresponsible actions by Washington, we stress: this is not our choice."
"We have frequently explained to the Americans ... why unilateral sanctions do not fit the standards of civilised relations between states," Lukashevich said.
Lukashevich did not describe any measures Moscow might impose in retaliation but said the Russian response would not necessarily mirror the U.S. sanctions.
The U.S. Senate Foreign Relations Committee is consulting the Obama administration on possible measures including visa bans and asset freezes against individuals, suspension of military cooperation, and economic sanctions.
Update 1:35 PM EST:
U.S. Secretary of State John Kerry told reporters in Kiev on Tuesday that the U.S. prefers military de-escalation, but if necessary, will "stand by the Ukrainian people" and 'defend' their territory by isolating Russia "diplomatically, politically and economically." Kerry took a swing at President Vladimir Putin, slamming him for sending military forces into the Crimea on false pretexts, charging, "Russia has been working hard to create a pretext to invade further."
In a shorter address from Washington, U.S. President Barack Obama urged Russia to de-escalate militarily and charged that Russia is violating international law.
The Guardian, which continues to offer live coverage on developments in Ukraine, offers this rundown of Tuesday's comments by both Kerry and Obama:
• Kerry flew to Russia’s backyard to accuse Moscow of blatantly lying about the security situation in Ukraine, the dynamics of the revolution there and the presence of Russian troops in the country.
• “Russia has been working hard to create a pretext for Russia to invade further,” Kerry said, after saying that Moscow is “hiding its hand behind falsehoods, intimidation and provocations.”
• Kerry said the United States wants Putin to de-escalate the situation by withdrawing troops fully within barracks and accepting international monitors.
• If Russia does not de-escalate, Kerry said, “then our partners will have absolutely no choice but to join us” in taking further steps to isolate Russia politically and economically.
• Kerry used vivid language to describe the scene at Independence square and to narrate what had happened there, with “shrouded vision in the clouds and the fog of the buildings from which the shots came.”
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• Obama expressed disbelief at Putin’s explanation of the Russian position. “Mr Putin can throw a lot of words out there, but the facts on the ground indicate” a Russian infringement, Obama said.
• Russia’s conduct is “not based on actual concern for Russian nationals or Russian speakers... but Russian seeking to exert force on a neighboring country,” Obama said.
• “The way some of this has been reported there is a suggestion that Russia’s actions have been clever, but this has not been a sign of strength,” Obama said.
• Both Obama and Kerry called on Russia to wait for Ukrainian elections in 90 days and to let Ukrainians ratify their new government or not.
Russia's President Vladimir Putin addressed the world in a televised press conference Tuesday saying the tense situation in Crimea would not descend into all out war in Ukraine and that military force would only be used as a last resort.
"We will not go to war with the Ukrainian people. If we do take military action, it will only be for the protection of the Ukrainian people," said Putin, who described those living in the former Soviet block as Russia's "brothers and sisters."
He continued: "Ukraine is not only our closest neighbor it is our fraternal neighbor. Our armed forces are brothers in arms, friends. They know each other personally. I'm sure Ukrainian and Russian military will not be on different sides of the barricades but on the same side. Unity is happening now in the Ukraine, where not a single shot has been fired, except in occasional scuffles."
Those "scuffles" include some tense moments also reported Tuesday between Ukrainian soldiers and those dressed in Russian fatiques who have taken up positions at key points in Crimea, including military installations.
Putin's remarks came as U.S. Secretary of State arrived in Kiev to offer the newly declared government there the full backing of the United States, including financial and political support. The U.S. has also said that it is preparing a set of diplomatic and financial sanctions against Russia for what it calls hostile interference in the Crimean peninsula, but Putin said such moves by the Obama administration would be "counter-productive."
According to the Guardian, "A Kremlin aide said on Tuesday that if the US were to impose sanctions, Moscow could be forced to drop the dollar as a reserve currency and refuse to pay off any loans to American banks."
Though called a "popular revolt" by the U.S. and many of its EU and British allies, Putin said the overthrown of the democratically-elected Viktor Yanukovych more than two weeks ago was nothing less than "an "anti-constitutional coup and a military seizure of power." Despite that assertion, Putin acknowledged the existence of a new reality in Ukraine.
"Yanukovych has no political future now, I have told him that," Putin said. He characterized the acting parliament now in Kiev as "partly legitimate" but questioned the authority of some who have asserted power.
In terms of Crimea, Putin said he is not interested in war, but preserving stability and security for those in Ukraine who feel threatened by the new authorities.
"If mayhem breaks out in eastern Ukraine and people ask for help, Russia reserves the right to react, but this is a last resort," Putin said at the press conference. "It's a humanitarian mission, we don't aspire to enslave anyone."