Ukrainian President Petro Poroshenko is accusing Russia of invasion after deploying its troops in eastern Ukraine.
Petro Poroshenko’s remarks came as pro-Russian rebels opened a new front in the south by seizing the coastal town of Novoazovsk.
NATO says it has detected a significant increase of Russian arms being supplied to the rebels over the past two weeks.
Russia has denied that its forces have crossed Ukraine’s border. At least 2,119 people have been killed in four months of fighting.
The UN Security Council is to hold an emergency meeting in New York on August 28 at 18:00 GMT to discuss the crisis.
NATO Brigadier General Niko Tak said there had been a “significant escalation in the level and sophistication of Russia’s military interference in Ukraine” over the past two weeks.
“[NATO has] detected large quantities of advanced weapons, including air defense systems, artillery, tanks, and armored personnel carriers being transferred to separatist forces in eastern Ukraine,” he said.
President Petro Poroshenko is accusing Russia of invasion after deploying its troops in eastern Ukraine
“Russia is reinforcing and resupplying separatist forces in a blatant attempt to change the momentum of the fighting, which is currently favoring the Ukrainian military.”
More than 1,000 Russian troops are operating inside Ukraine, both supporting the separatists and fighting on their side, according to NATO.
However, Russia’s ambassador to the EU, Vladimir Chizhov, said NATO had “never produced a single piece of evidence” for its accusations. He said the only Russian soldiers on Ukrainian soil were 10 paratroopers captured earlier this week.
On August 28, Ukrainian President Petro Poroshenko called an emergency meeting with his security council to discuss “the sharp aggravation of the situation in Donetsk region… as Russian troops were actually brought into Ukraine”.
“The situation is extremely difficult, but it is manageable enough for us not to panic and continue calculating our actions,” he told security chiefs.
Meanwhile Prime Minister Arseny Yatsenyuk said Russia had “unleashed a war in Europe”, adding that the world should take “effective steps”.
Government forces had made significant advances against the separatists in recent weeks, but these gains seem in doubt with rebels now operating in two distinct areas of Donetsk region.
Separatist leader Alexander Zakharchenko told Russian TV that between 3,000 and 4,000 Russian citizens were fighting in their ranks.
He said many of the Russians were former service-people or current service personnel on leave, insisting that all were volunteers.
Ukraine’s security and defense council confirmed reports that Novoazovsk had been captured by the rebels, whom they described as “Russian troops”.
It said it had withdrawn its forces to save lives, and that Ukrainian soldiers were now reinforcing the defenses of the strategic port city of Mariupol.
The port has until now been peaceful and cut off from rebel positions.
Pro-Russian fighters have been trying for weeks to break out of an area further north in the Donetsk region where they are almost encircled.
Analysts say the separatists could also be seeking a land link between Russia and Crimea, which would give them control over the entire Sea of Azov.
Russia annexed the Black Sea peninsula of Crimea from Ukraine in March.
Russia has admitted for the first time that ten Russian soldiers captured in eastern Ukraine had crossed the border “by accident”.
Ukraine said ten Russian paratroopers had been captured and has released video interviews of some of the men. One is quoted as saying “this is not our war”.
The incident comes ahead of a key meeting between the Ukrainian and Russian presidents.
Ukraine’s President Petro Poroshenko and his Russian counterpart Vladimir Putin are at a summit in Minsk in Belarus.
More than 2,000 people have died in months of fighting between Ukrainian forces and separatists in the Donetsk and Luhansk regions.
The two regions declared independence from Kiev following Russia’s annexation of the southern Crimean peninsula from Ukraine in March.
A Russian defense ministry source was quoted by the Russian news agency RIA Novosti as saying: “The soldiers really did participate in a patrol of a section of the Russian-Ukrainian border, crossed it by accident on an unmarked section, and as far as we understand showed no resistance to the armed forces of Ukraine when they were detained.”
The source also said that some 500 Ukrainian servicemen had crossed the border at various times, adding: “We did not give much publicity to that. We just returned all those willing to return to Ukrainian territory at safe places.”
Ukraine’s security service has captured ten Russian paratroopers near the village of Dzerkalne (photo Ukraine Defence Ministry)
Ukraine’s security service said its military had captured the 10 Russian paratroopers near the village of Dzerkalne, about 30 miles south-east of the rebel-held city of Donetsk and about 15 miles from the Russian border.
Ukrainian military spokesman Andriy Lysenko said: “This wasn’t a mistake, but a special mission they were carrying out.”
A Ukrainian television report that carried the interviews with the men said they were from the 331st regiment of the 98th Svirsk airborne division.
It quoted one man, named as Sgt Andrei Generalov, as saying: “Stop sending in our boys. Why? This is not our war. And if we weren’t here, none of this would have happened.”
Another man, named as Ivan Milchakov, says he is based in the Russian town of Kostroma.
“I did not see where we crossed the border. They just told us we were going on a 70km march over three days,” he said.
“Everything is different here, not like they show it on television. We’ve come as cannon fodder.”
Russia has repeatedly denied Ukrainian and Western accusations that it is supporting the rebels.
On Monday, Ukraine said an armored column had crossed the border into south-eastern Ukraine, sparking clashes near Novoazovsk.
The summit in Minsk is also being attended by senior officials from the European Union which, along with the US, has imposed sanctions on Russia for failing to rein in the separatists.