European Union leaders are poised to slap additional sanctions on Russia because of what they condemn as a continued destabilization of Ukraine and a deepening military involvement by Moscow.
The 28-nation bloc's heads of state and government on Saturday were also set to discuss possible further assistance for Ukraine as the country's president, Petro Poroshenko, was expected to brief the leaders in Brussels on the situation.
Additional EU sanctions would aim at inflicting more economic pain in hopes of changing what is viewed as Russian meddling in Ukraine.
Meanwhile, Ukrainian forces say one of their fighter jets has been shot down by a Russian missile in combat against Russia-backed separatists in eastern Ukraine.
A brief statement posted Saturday on the Facebook page for the so-called operation against separatists said the Su-25 was hit on Friday, and that the pilot ejected and was uninjured. The statement said the plane was hit by a missile from a Russian launcher, but did not give more detail.
The fighting in eastern Ukraine between the military and Russian-backed separatists has left almost 2,600 people dead, according to U.N. figures. NATO said this week Moscow has slipped at least 1,000 Russian soldiers and much heavy weaponry into Ukraine.NATO estimates at least 1,000 Russian soldiers are in Ukraine and Ukraine claimed this week that Russian tanks and armored vehicles entered the country as rebels open a new front along the Azov Sea coast. Russia consistently denies both that its forces are in Ukraine and allegations that it is supplying the rebels.
In a blunt statement Friday, NATO condemned Russian military action in Ukraine, saying its troops have "illegally crossed the border" as part of a "dangerous pattern over many months" to destabilize its neighbor.
NATO Secretary General Anders Fogh Rasmussen, speaking after a meeting of the NATO-Ukraine commission, was unsparing in his criticism of Russia, charging that its forces were directly engaged in military operations inside Ukraine, continued to supply separatists with tanks, armored vehicles, artillery and rocket launchers and had fired on Ukraine from both Russian territory and within Ukraine itself.
"This is a blatant violation of Ukraine's sovereignty and territorial integrity," Rasmussen said. "It defies all diplomatic efforts for a peaceful solution."
Russia has denied its troops have operated inside Ukraine, despite NATO's release of satellite images on Thursday showing what the Western alliance said were Russian tanks, armored personnel carries and artillery moving around in eastern Ukraine.
The Associated Press reported that Russian-backed separatists held control Friday of the coastal town of Novoazovsk on the new front in the Ukraine conflict. The separatists also announced their intention to keep pushing west toward a major port city. None of the half-dozen tanks seen by Associated Press reporters in the town of about 12,000 people bore Russian markings, but the packaging on their field rations said they were issued by the Russian army.
Speaking at a youth forum in central Russia on Friday, President Vladimir Putin said Russia "is far from getting involved into any large-scale conflict."
"We do not want and do not intend to do this," Putin said, but noted that Russia is one of the world's most powerful nuclear nations. "This is a reality, not just words," the Russian leader said.
He also declared that Russia is building up its nuclear and military arsenal.
"We must always be ready to repel any aggression against Russia and our partners should always be aware that no matter in which condition their governments may be or which foreign policy concepts they may pursue, it is better not to come against Russia as regards a possible armed conflict," Putin said, the Russia news agency ITAR-TASS reports.
Regarding Ukraine, Putin said Kiev insists it wants talks to resolve the crisis but keeps pushing its military to try to quickly establish order there.
"This is not happening and it is important to realize it and to force the Ukrainian authorities to start thorough negotiations, not only on technical issues, which are certainly of extreme importance, but on substantive (issues) as well," Putin said.
In a separate news conference in Moscow, Russian Foreign Minister Sergei Lavrov dismissed Western claims of Russian intervention in Ukraine as more "wild guesses."
"There have been reports about satellite imagery exposing Russian troop movements. They turned out to be images from videogames. The latest accusations happen to be much the same quality," he said, according to the state-run Russian news agency RT.com.
Nearly 2,600 people have died in clashes between Ukrainian armed forces and Russian-back rebels since April, according to a new U.N. report.
Rasmussen, the NATO chief, said in his remarks that the Western alliance would help Ukraine reform and modernize its armed forces "with a view to make them stronger" and could put forward additional assistance at the NATO summit in Wales next week that will be attended by Ukrainian President Petro Poroshenko.
The White House says the Ukrainian president will meet with President Obama in Washington Sept. 18.
European Union foreign ministers will consider harsher sanctions on Russia during their meeting in Milan on Friday. The EU and the U.S. have already imposed sanctions on Russian institutions and officials, while Russia has responded by banning food imports.
In Kiev, Ukrainian Prime Minister Arseniy Yatseniuk said the government has offered a bill that would cancel the country's non-aligned status as a first step toward joining the European Union and, eventually, NATO.
The crisis began last November when thousands of protesters took to the streets of Kiev after pro-Russian prime minister Viktor Yanukovych, who was later ousted and fled the country, refused to sign an agreement that would have integrated Ukraine's economy closer to the European Union, making it less dependent on Russia.
Ukraine charged that Russian military convoys entering Ukraine in the past week opened a new front in the fighting that had largely focused on territory around Donetsk and Luhansk in the industrialized eastern regions. Rebels have declared the two areas as independent republics.
Earlier, in Moscow, Putin issued a rare, direct address to the separatists, lauding their latest offensive and calling on rebels to open a humanitarian corridor to allow encircled Ukrainian army units to withdraw and return to their families.
Putin said the Ukrainian soldiers "did not participate in the military operation of their own volition" and were only following orders. He asked for their release, "so as to avoid any needless loss of life."
Separatist leader Alexander Zakharchenko later told Russian TV that his fighters had agreed to Putin's request, provided that the Ukrainian troops turn over heavy weapons and ammunition.
Contributing: William Cummings; the Associated Press