"There is mounting evidence that shows a spread of Russian military forces near the border with Ukraine," Xinhua quoted White House spokesman Josh Earnest as saying Friday.
"We will not accept any use of Russian military forces under any pretext in eastern Ukraine," Earnest said, adding that the US is closely monitoring the situation.
Russia said Friday that it was perplexed over recent Western allegations that it was increasing its military presence near the Ukrainian borders.
"In this case, any buildup of troops is out of the question," Kremlin spokesman Dmitry Peskov said, adding that there were indeed "measures to enhance security along the borders" that were in line with orders from President Vladimir Putin.
On June 7, Putin ordered the State Border Service to undertake appropriate steps to tighten control on the Russian-Ukrainian borders to curb illegal crossings.