Reuters / Ivan Sekretarev
Russia's President Vladimir Putin listens to his spokesman Dmitry Peskov before a meeting with his Finnish counterpart Sauli Niinisto at the Bocharov Ruchei state residence in Sochi August 15, 2014.
Susan Rice, President Barack Obama's national security adviser, said late Monday night that Russia's military "incursions" into Ukraine represented a "significant escalation."
"Repeated Russian incursions into Ukraine unacceptable. Dangerous and inflammatory," she wrote on Twitter. She accused Russia of using artillery, air defence systems, tanks, and troops in the "incursions."
Her comments came after Ukraine said it had detained 10 Russian paratroopers, who Moscow said had crossed the border "mostly by mistake."
Geoffrey Pyatt, the U.S. ambassador to Ukraine, wrote on Twitter that Russia's recent moves in Ukraine "may indicate that a Russian counteroffensive is underway" in southeastern Ukraine, where fighting between the Ukrainian army and pro-Russian separatists have raged for months.
Russia has shown no signs of slowing down, four days after it incurred the wrath of NATO, the European Union, and the United States when it sent what it called a convoy carrying humanitarian aid across the border without Ukraine's permission. The White House accused Russia of painting military trucks to look like civilian vehicles.
Russian Foreign Minister Sergei Lavrov said Monday it plans to send another aid convoy in the coming days. He said he had notified Kiev, which denied any knowledge of the plan. Meanwhile, a top Ukrainian official said Monday that a new column of Russian tanks and armored vehicles entered into Ukraine.
It all comes as Putin and Ukrainian President Petro Poroshenko are meeting in Minsk, Belarus, along with other European leaders. It is the first time the two will have met face-to-face since June.