"It is important for us to take further steps sending a message to Russia that these kinds of destabilising activities taking place in Ukraine has to stop," Obama said at a press conference in Malaysia. (Agencies)
Obama spoke a day after G7 nations said that they would impose new sanctions on Russia within days, accusing Moscow of doing nothing to honour an agreement forged in Geneva aimed at easing tensions in Ukraine.
"So long as Russia continues down a path of provocation rather than trying to resolve this issue peacefully and de-escalate it, there are going to be consequences and those consequences will continue to grow," he said.
Ukrainian Prime Minister Arseniy Yatsenyuk claimed Russia had violated his country's airspace seven times overnight Friday with an aim "to provoke" Ukraine into starting a war.
US Secretary of State John Kerry told his Russian counterpart Sergei Lavrov that Washington was concerned about "provocative" troop movements along Russia's border with Ukraine and its support for the separatists, which he said "are undermining stability, security and unity in Ukraine".
Yatsenyuk cut short a visit to the Vatican as concern grew that the tens of thousands of Russian troops conducting military drills on the border could soon be ordered to invade.
But Moscow denied any transgression by its warplanes, with Lavrov calling for "urgent measures" to calm the crisis, which has plunged East-West relations to their lowest point
since the Cold War.
A Western diplomat warned: "We no longer exclude a Russian military intervention in Ukraine in the coming days."
The diplomatic source noted that Russia's UN envoy, Vitaly Churkin, "has been recalled urgently to Moscow" for consultations.
Meanwhile, international efforts were under way to secure the release of a 13-member mission from the Organization for Security and Co-operation in Europe held hostage by pro-Russian militants in the flashpoint city of Slavyansk.
"It is important for us to take further steps sending a message to Russia that these kinds of destabilising activities taking place in Ukraine has to stop," Obama said at a press conference in Malaysia.