U.S. President Barack Obama says Russia must pull back its troops from the Ukrainian border and begin negotiating to defuse tensions.
In an interview with CBS, Mr. Obama said Russia's military moves may be no more than an effort to intimidate Ukraine. But he added that Russia may have "additional plans."
On Thursday, the head of Ukraine's national security council said Russia has close to 100,000 troops along Ukraine's borders in the north, south, and east. He said Russian forces are in full readiness to strike.
Russia says the soldiers are involved in "springtime exercises" and has assured the United States they will not cross the border.
Western experts believe the number of Russian forces near eastern and southern Ukraine is close to 30,000.
Meanwhile, a top Russian security official told President Vladimir Putin on Friday that there has been "a sharp increase in external threats to the state."
Alexander Malevany, the Federal Security Service's (FSB) counter-terrorism director, also told Mr. Putin that what he called the "legitimate desire of the peoples of the Crimea and eastern regions of Ukraine to be together with Russia" had aroused "hysteria" among the United States and its allies.
Malevany accused the U.S. and its allies of taking measures "to disrupt Russia's social, political and economic life."
Also Friday, Russia described as "counterproductive" a U.N. resolution that refuses to recognize its annexation of the Crimean peninsula from Ukraine. Russia's Foreign Ministry said the U.N. General Assembly resolution will only complicate efforts to settle Ukraine's internal political crisis.
The ministry accused Ukraine of seeking to distract from domestic tensions by blaming its problems on Russia.
The U.N. General Assembly passed the non-binding resolution on Thursday, with 100 countries in favor, 11 opposed and 58 abstaining.
Crimea's majority Russian residents voted to break away from Ukraine and join Russia in a referendum earlier this month that Western powers deemed illegal.
Ousted Ukrainian president Viktor Yanukovych called Friday for referendums to determine the status of all Ukrainian regions. He said only a nationwide referendum and not an early presidential election can stabilize Ukraine and preserve its sovereignty and integrity.