U.S. President Barack Obama says Ukraine does not need additional military assistance to help fight pro-Russian separatists but an invasion by Russia would raise "a different set of questions."
Obama's comments came after NATO said on August 6 that Moscow had increased its forces along the border with eastern Ukraine to some 20,000 troops.
Obama said Ukraine is fighting separatists "who can't match the Ukrainian Army."
Ukrainian officials have urged Washington to provide lethal aid to Kyiv.
NATO said that some Russian military vehicles on the border have been marked as "peacekeeping" or "humanitarian," causing worry Moscow might use a humanitarian pretext to invade.
Meanwhile, U.S. Vice President Joe Biden spoke via phone with Ukrainian President Petro Poroshenko, who said Moscow is already involved in the conflict, firing artillery at Ukrainian forces from Russia.
Based on reporting by Reuters, AP