National Security and Defense Council spokesman Andriy Lysenko told ICTV that the number of Russian forces is believed to be between 10,000 and 15,000.
With tension mounting, Ukrainian forces are engaged in fighting with Russian troops. On Monday, Lysenko told reporters that in the last 24-hours, "Anti-Terrorist Operation forces engaged 37 times with Russian mercenaries and terrorists" in the eastern regions of Donetsk and Luhansk.
Ukraine's National Guard described a battle in Rozdolne on Monday. "Four trucks came to the roadblock with white flags and inscriptions 'Children,' armed people jumped from the trucks and opened fire on the Ukrainian military." Despite Russia's attempts to conceal the troops and rebels, the National Guard reported, "The attackers have severe losses of manpower and equipment. As a result, four trucks of terrorists were neutralized."
Russia, meanwhile, insists that it has not sent troops into Ukraine, despite mounting evidence to the contrary.
U.S. Ambassador to the United Nations Samantha Power circulated the below satellite imagery of Russian artillery moving within Ukraine's borders.
Russia says US sldn't interfere in internal affairs of Ukraine. Interesting. Is this what noninterference looks like? pic.twitter.com/rs8dCD0u0j— Samantha Power (@AmbassadorPower) August 28, 2014
Following a phone call with German Chancellor Angela Merkel last week, President Barack Obama said, "We agree -- if there was ever any doubt -- that Russia is responsible for the violence in eastern Ukraine" and remarked that "the new images of Russian forces inside Ukraine make that plain for the world to see."
Obama is scheduled to join fellow NATO representatives at a summit in Wales this week, where they are expected to discuss the crisis in Ukraine as well as the threat posed by the Islamic State in Iraq and Syria.