Ukrainian President Petro Poroshenko accused Russia on Monday of “direct and open aggression” against his country.
Speaking at a military academy in Kyiv, he said Russia's direct involvement in the war against the separatists in eastern Ukraine had tipped the balance on the battlefield and was the main reason for recent reversals.
"Direct and open aggression has been launched against Ukraine from a neighboring state. This has changed the situation in the zone of conflict in a radical way," he said.
Meanwhile in London, British Prime Minister David Cameron on Monday called the presence of Russian troops on Ukrainian soil “unjustified and unacceptable.”
"Russia appears to be trying to force Ukraine to abandon its democratic choices at the barrel of a gun," Cameron told parliament, warning Moscow its relationship with the rest of the world would be "radically different" in future if it continued with its current policy on Ukraine.
Separately, Polish Prime Minister Donald Tusk said what is happening in Ukraine should be seen as a war, cautioning that September 1939 not be repeated – a reference to Poland being invaded by Nazi Germany and subsequently attacked by the Soviet Union.
Speaking at a ceremony marking the 75th anniversary of the beginning of World War II, Tusk said there was still time to stop those “for whom violence, force [and] aggression are again becoming an arsenal of political activity.”
NATO forms rapid reaction force
NATO Secretary General Anders Fogh Rasmussen has announced a new "very high readiness force" in response to what he says is Russian aggression and intervention in Ukraine.
Rasmussen said in Brussels Monday that the force would include several thousand troops ready to go where needed on very short notice. He said they would have air, sea and special forces support.
The force is part of what he called a Readiness Action Plan that would make NATO more agile than ever, and would represent a more visible presence in Eastern Europe.
Details of the plan will be presented at the NATO summit in Wales later this week.
Rasmussen said the summit is taking place in what he said is a changed world. He said NATO has to adapt to the Russian attitude that Europe is an adversary.
EU threatens new sanctions
Over the weekend, European Union leaders, meeting in Brussels, called on Russia to "immediately withdraw all its military assets and forces from Ukraine" or face a new round of sanctions within a week.
Chancellor Angela Merkel acknowledged on Monday that enacting further punitive measures against Russia could hit the German economy, but said that doing nothing in response to Moscow's aggression in Ukraine was “not an option.”
“I have said that [sanctions] can have an impact, also for German companies,” Merkel told a news conference in Berlin. “But I have to say there is also an impact when you are allowed to move borders in Europe and attack other countries with your troops,” she added.
“Accepting Russia's behavior is not an option," said Merkel.
Russian President Vladimir Putin, during a visit to Siberia, urged the EU on Monday to show "common sense" and not to resort to mutually destructive sanctions, in his first reaction to the threat of additional punitive measures over Ukraine.
Continued Russian denials
Russia has repeatedly denied it has any troops in Ukraine. Also, Foreign Minister Sergei Lavrov repeated Moscow's position Monday that it has never offered material support to the rebels. He also dismissed Ukraine's demand the rebels first disarm and called again for an unconditional cease-fire.
Lavrov said that there will be no military intervention and that Russia supports only a peaceful resolution to the crisis. Everything Russia does, he said, is aimed at promoting a political approach. But unfortunately, he added, Western countries are blind to the situation, and everything Kyiv authorities declare and do receives support in the West.
Meanwhile, pro-Russian separatist leader Andrei Purgin says he is ready to discuss a cease-fire and prisoner exchange at peace talks in Minsk.
The first round ended Monday with no major progress. But Purgin, leader of a self-proclaimed Donetsk people's republic and separatists aligned with him say they are willing to remain part of Ukraine if they are granted more autonomy.
There has been no official reaction from Kyiv.
Situation on the ground
Ukrainian security authorities say a column of Russian tanks backed-up separatist rebels in the east as they pushed government forces from Luhansk airport.
Ukraine National Security and Defense Council spokesman Colonel Andriy Lysenko says Ukrainian troops were forced to withdraw after coming under heavy fire.
He said that judging from the precision of the strikes, the shelling was by professional artillerymen of the Russian Federation.
The Ukrainian spokesman said no less than 1,600 Russian soldiers are operating inside Ukraine to support the rebels.
Moscow says Kyiv is claiming Russian troops are in Ukraine as an excuse to explain its recent military losses.
Meanwhile, NATO Secretary-General Ander Fogh Rasmussen told reporters in Brussels Monday that Ukraine's next parliament, to be elected late October, will likely change the country's "non-aligned status," a possible first step toward applying to join the Western military alliance.
School year starts
Despite raging conflict in the area, children in the eastern Ukrainian city of Slovyansk attended their first day of school on Monday.
Young students in uniform and traditionally embroidered items of clothing attended a school assembly, handing flowers to their teachers, before walking to their classes in groups.
Head teacher at the school, Aleksandr Pastukhov, said he hoped the new school year would herald peace.
Asked to what extent the school had been affected by ongoing tensions in the east, Pastukhov said some 38 children had not returned for classes and their families could not be reached by telephone, suggesting many had fled Slovyansk.
"Five teachers have left our so called school family. But as of today we have found replacements," he added.
Daniel Schearf contributed to this report from Kyiv. Some information provided by Reuters, AP and AFP.