In addition to denying that Russian military forces have recently invaded Ukraine, Russian President Vladimir Putin compared Kiev's attack on cities that are occupied by pro-Russian rebels to the Nazi invasion of the Soviet Union.

Over the past week, Russia has been accused of sending at least 1,000 soldiers and military armor to Ukraine in order to help bolster the flagging separatist insurrection.

Putin continues to deny that Russian military is aiding the separatist movement, despite alarming evidence that Russia has effectively invaded Ukraine. He also claims that the Ukrainian army is targeting residential towns and cities, just like Nazi soldiers did in World War II.

"Small villages and large cities surrounded by the Ukrainian army, which is directly hitting residential areas with the aim of destroying the infrastructure," he said while speaking at a pro-Kremlin youth camp on Friday, according to Reuters. "It sadly reminds me the events of the Second World War, when German fascist ... occupiers surrounded our cities."

He went on to boast that Russia is a strong nuclear power, warning that "Russia's partners ... should understand it's best not to mess with us," reports The Guardian.

He also stressed that his government is not responsible for the Ukraine invasion.

"We do not want and do not intend to do this," the Russian leader said. "This is a reality, not just words."  

In response to accusations against Russia, NATO ambassadors condemned Russia of a "blatant violation" of Ukraine's sovereignty.

"Despite Moscow's hollow denials, it is now clear that Russian troops and equipment have illegally crossed the border into eastern and south-eastern Ukraine," NATO Secretary General Anders Fogh Rasmussen said.

"This is a blatant violation of Ukraine's sovereignty and territorial integrity. It defies all diplomatic efforts for a peaceful solution."

President Barack Obama also made a statement criticizing Russia for overstepping its bounds.

"Russia has deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine," Obama said.