Moscow is not expecting NATO forces to intervene if it grabs Mariupol.
Mariupol is on the road to Crimea, but taking the city is far beyond the rebels' reach without Putin's troops.
The failure of NATO's leaders to check Putin's advance is making a mockery of the alliance's purpose.
Some volunteer groups have doubled in size since the start of the conflict in Ukraine.
NATO's chief pointed to a "steady flow of heavy equipment, tanks, artillery, ammunition, air defense systems and a lot of training."
The battered seaside town of Shyrokyne stands between the Russian-armed rebels and Russian-occupied Crimea.
U.S. and Ukrainian official have both reported that both Russian troops and weaponry are gathering on the border.
Washington says Moscow training pro-Russian rebels and quantity of air defence equipment in eastern Ukraine "highest since August".
The U.S. accused Russia of building up air defense systems inside eastern Ukraine and of involvement in training exercises of pro-Russian separatists.
"This is how it was in World War II in America.Ordinary people are ready to die for their country."
Pro-Russian politicians in Kiev are dying in highly suspicious circumstances.
Vessels led by anti-submarine ship on way to northern Atlantic for defense drills, says Russian navy.
Witnesses saw a soldier shot dead at point-blank range by a separatist commander.
Writing in a joint declaration, the defense ministers of Sweden, Norway, Finland, Denmark and Iceland said Northern Europe must prepare for possible crises or incidents because ofRussia.
Separatist forces called back from leave to mount massive offensive this month or next.
He aims to overthrow the entire post-Cold War order in Europe.
By playing nice on Iran, Russia hopes to strengthen its hand on Ukraine.
Former NATO chief urges U.S. to send "lethal aid" to Kiev.