The US has warned Russia it will face further sanctions if pro-Russia militia in eastern Ukraine do not disarm, as the Kremlin admits reinforcement troops have been sent to the border.
Barack Obama said the US could take further economic and diplomatic action against Moscow if an international agreement to calm tensions in Ukraine is not implemented.
Russia, Ukraine, the US and the European Union agreed on Thursday to a series of steps to "de-escalate" the crisis in Ukraine.
But many of the groups that took over government buildings in a bid to declare independence in the largely Russian-speaking part of eastern Ukraine have yet to leave, saying they will only do so if Ukraine's government steps down too.
US Secretary of State John Kerry told Russian foreign secretary Sergei Lavrov late on Friday that the "the next few days would be a pivotal period".

The EU has also indicated it will meet to discuss further sanctions if Russia does not act to make the militias disarm.
But earlier, the Kremlin had indicated Russia was not going to be bossed around, issuing a veiled threat.
Kremlin spokesman Dmitry Peskov told Russian television: "Our Western colleagues are trying to push responsibility towards our side. But it must be underlined: it is a collective responsibility."
"We have troops in different regions, and there are troops close to the Ukrainian border.
"Some are based there, others have been sent as reinforcements due to the situation in Ukraine," he added.
 US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov
 US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov  The White House said it was watching Moscow to see if it is holding up its end of the deal.
US National Security Advisory Susan Rice said: "We expect and we will be watching whether Russia does or does not uphold its responsibility to use its very considerable influence to restrain and withdraw those irregular militia from the buildings and spaces that they've occupied."
Pro-Russians occupying public buildings in 10 eastern Ukrainian cities have been told to leave in the next few days or face "more concrete actions" from the interim government.
Ukraine's Foreign Minister Andriy Deshchytsia gave no details of the likely actions, but said: "Hopefully, if those people are ready to leave the buildings, to surrender weapons, today, tomorrow, so we can encourage the OSCE (European security) mission to negotiate, to mediate and implement this.
"But if this will not start in a few days, I think that after Easter there will more concrete actions."
Kiev had previously declared the separatists as terrorists and sent troops to remove them, but had been unable to.
The separatists want a referendum on union with Russia, similar to the one held in Crimea that ultimately led to its annexation by Russia.
Denis Pushilin, a leader of the self-appointed Donetsk People's Republic, said insurgents do not recognise the Ukrainian government as legitimate.
Sky's Katie Stallard, in Donetsk, said there was no sign of anyone there acting on the deal reached in Geneva at talks attended by the US, Russia, the European Union and Ukraine.
She added: "None of those at the talks directly represented those on the ground, particularly those occupying the buildings."
Meanwhile, Europe's Baltic states are in talks to set up their own Russian-language television channel in a bid to counter the deluge of propaganda aimed at their ethnic Russian populations by Moscow-backed media.
 The Bluefin-21 submersible has already carried out six descents
 The Bluefin-21 submersible has already carried out six descents   The Chinese ship Hai Xin 01 conducts a search in the southern Indian Ocean
 The Chinese ship Hai Xin 01 conducts a search in the southern Indian Ocean   Family members of missing passengers wait for news at a gym in Jindo
 Family members of missing passengers wait for news at a gym in Jindo   A crane arrives at the scene
 A crane arrives at the scene   Lee Joon-Seok was not at the helm when the ship began listing
 Lee Joon-Seok was not at the helm when the ship began listing   Anxious relatives take part in a Buddhist ritual
 Anxious relatives take part in a Buddhist ritual   Mr Nel, who grilled Pistorius for five days, is quizzing a forensic expert
 Mr Nel, who grilled Pistorius for five days, is quizzing a forensic expert  
 Ms Steenkamp was shot dead at Pistorius' home on Valentine's Day last year
 Ms Steenkamp was shot dead at Pistorius' home on Valentine's Day last year  

 An armed personnel carrier in Slavyansk
 An armed personnel carrier in Slavyansk  
 US President Barack Obama has threatened Russia with further sanctions
 US President Barack Obama has threatened Russia with further sanctions   Armed men stand guard outside a building occupied by pro-Russia militia
 Armed men stand guard outside a building occupied by pro-Russia militia   John Kerry meets with Catherine Ashton in Geneva
 John Kerry meets with Catherine Ashton in Geneva   Rescued passengers are brought ashore
 Rescued passengers are brought ashore   The ferry ran into difficulties 60 miles south of the Korean peninsula
 The ferry ran into difficulties 60 miles south of the Korean peninsula   95% of the ship is now submerged
 95% of the ship is now submerged   The ship, Sewol
 The ship, Sewol   459 people were on the ship when it started to sink
 459 people were on the ship when it started to sink   Rescued passengers are wrapped in blankets
 Rescued passengers are wrapped in blankets   34 rescue boats are at the scene
 34 rescue boats are at the scene   A mother reacts to seeing her son on the list of those rescued
 A mother reacts to seeing her son on the list of those rescued  
 HMS Echo is searching the southern Indian Ocean for the missing jet
 HMS Echo is searching the southern Indian Ocean for the missing jet   The Bluefin-21 can search between 20 and 40 square miles a day
 The Bluefin-21 can search between 20 and 40 square miles a day   Sunday's planned search area
 Sunday's planned search area  

