This post was updated at 4:50 p.m. ET.
Russia's parliament has unanimously approved a request by President Vladimir Putin to authorize the intervention of Moscow's forces in Ukraine until "the normalization of the political situation" there. In response, Ukraine put its own forces on alert and warned that a Russian invasion would spark war between the two countries.
Russia's Federation Council vote came quickly after a formal appeal from Putin, but seemed to only rubber-stamp what has already occurred: Ukraine's defense minister said thousands of additional Russian troops are already in the Crimea in what appeared to be a move aimed at maintaining Kremlin access to the strategic peninsula, where Russia's Black Sea fleet is based.
After the vote, Putin spokesman Dmitry Peskov was quoted by Reuters as saying:
"[The] president [now] has received the full arsenal of means needed to resolve the situation, in terms of using [military] forces and in terms of taking decisions about [withdrawing] the head of our diplomatic mission in the United States."
Although the additional Russian force appeared, at least for now, to be limited to the Crimean peninsula, Putin's appeal, which called for authorizing force in the whole of the country, seemed to set up the possibility of a wider intervention.
Initially, Ukraine said 6,000 additional Russian troops had entered the country illegally, but speaking at a U.N. Security Council emergency meeting on Saturday, ambassador Vitaly Churkin said 15,000 Russian troops in Crimea.
At the same meeting, U.S. Ambassador Samantha Power said it was time for the intervention to end. "Actions speak louder than words," she said in a blunt statement to the Council.
She called the Russian action "dangerous and destabilizing" and "without legal basis."
Meanwhile, Russian troops and mysterious balaclava-clad soldiers are actively patrolling key installations and government buildings in Crimea. The military moves follow the installation this week in Crimea of Sergiy Aksyono, a pro-Russian leader, and the ouster a week ago of pro-Russian President Viktor Yanukovych.
After Yanukovych fled to Russia last week, Aksyono asked Moscow for help in "stabilizing the region." On Saturday Aksyono claimed Saturday that he's coordinating directly with troops from Russia's Black Sea fleet and that those forces were guarding government buildings.
Aksyono's election has been deemed illegal by Kiev.
Russia's Interfax news agency quotes Ukraine's Prime Minister Arseny Yatsenyuk as demanding that Russia "recall their forces, and return them to their stations."
"Russian partners, stop provoking civil and military resistance in Ukraine," Yatsenyuk said in an apparent reference to pro-Russian partisans in eastern Ukraine.
Later, after a three-hour meeting with Ukraine's security and defense chiefs, Acting President Oleksander Turchinov said the country's military had been placed on combat alert. Yatsenyuk said a Russian invasion would represent "the beginning of war and the end of any relations between Ukraine and Russia."
The U.N. Security Council met Saturday afternoon in an open, televised session to discuss the Ukraine crisis, a meeting that followed earlier closed-door consultations. The council heard speeches from a U.N. deputy secretary-general and several ambassadors, but did not take any action.
Yuriy Sergeyev, Ukraine's ambassador to the U.N., asked the Security Council, of which Russia is a member, to help safeguard its territorial integrity and "do everything possible now" to stop what he called Russia's "aggression."
Russia's ambassador to the United Nations, Vitaly Churkin, says Russian military movements are within the purview of existing security arrangements between Kiev and Moscow.
"We have an agreement with Ukraine on the presence of the Russian Black Sea fleet with a base in Sevastopol, and we are acting within the framework of that agreement," he told reporters after a closed-door meeting of the U.N. Security Council.
He said Ukraine needs "to refrain from conducting a hasty presidential election. They need to stop trying to intimidate other regions and other political forces."
U.N. Secretary General Ban Ki-moon has called "for the full respect and preservation of the independence, sovereignty and territorial integrity of Ukraine," spokesman Martin Nesirky said.
Meanwhile, President Obama's national security team met on Saturday, a senior administration official tells Reuters. They discussed options for a response to the crisis. Obama on Friday said he is "deeply concerned" by reports coming from the region and warned of unspecified "costs" for Russia from the U.S. and the international community.
RIA News Agency says that in a phone call between Obama and Putin, the Russian leader said his country reserves the right to protect its interests and those of Russian speakers in Ukraine.
The diplomatic shadow boxing is the latest in what many view as an outright Russian invasion of Crimea triggered by President Viktor Yanukovych's ouster last week. Yanukovych was forced out by a months-long protest calling on Ukraine to move away from Russia and align more closely with the West. Ukraine's interim government has promised fresh elections.
Reuters quotes Vitaly Klitschko, a senior Ukrainian politician and likely presidential candidate, as calling for a "general mobilization" of Ukrainian forces in the face of Moscow's intervention.
Germany, which depends heavily on Russia for its natural gas supply, also had cautionary words for Moscow. Foreign Minister Frank-Walter Steinmeier on Saturday said the situation in Crimea "had become considerably more acute."
"Whoever pours more oil onto the flames now, with words or actions, is consciously aiming for further escalation of the situation," he said. "Everything Russia does in Crimea must be in keeping with the sovereignty and territorial integrity of Ukraine, and treaties on Russia's Black Sea fleet."
"We are holding the Russian government to its public statements on this," Steinmeier said. "And this entails also that Russia provides without delay complete transparency over the movements of its troops in Crimea, as well as its goals and intentions behind these."
U.K. Foreign Secretary William Hague has also expressed concern to his Russian counterpart, Sergei Lavrov.
"I told Minister Lavrov that Britain supports the Ukrainian government's request for urgent consultations in accordance with the 1994 Budapest Memorandum signed by the U.K., U.S., Russia and Ukraine," Hague said, adding that London has "summoned the Russian ambassador to the Foreign Office to register our deep concerns."
The EU foreign policy chief, Catherine Ashton, said Russia's decision was unwarranted.
"I therefore call upon the Russian Federation not to dispatch such troops but to promote its views through peaceful means," Ashton said in a written statement on Saturday.
She said she would meet with Lavrov after meeting with EU foreign ministers on Monday.
NATO ambassadors were set for a meeting on Sunday to discuss Ukraine.