MARIUPOL, Ukraine — As concern over Russia’s actions in Ukraine grows, European leaders are meeting Saturday in Belgium to discuss possible new sanctions against Moscow.

The situation in Ukraine “has worsened considerably” European Commission President Jose Manuel Barroso said in a joint press conference with Ukrainian President Petro Poroshenko in Brussels.

“The opening of new fronts and use of regular Russian forces is not acceptable and represents a grave transgression,” Barroso said.

Poroshenko also said the situation had worsened since Wednesday and that thousands of foreign troops and hundreds of foreign tanks are now on Ukrainian territory.

“Ukraine now is a subject for foreign military aggression and terror,” he said. But European leaders have shown their support for Ukraine, he said, and Russian aggression will not go unanswered.

Barroso said he had urged Russia’s President Vladimir Putin to “change course” in a phone call Friday.

“No one’s interest is served by new wars on our continent, no one’s interest is served by confrontation,” he said. “This is simply not the way that responsible, proud nations should behave in the 21st century.”

Putin: Russia is powerful nuclear nation

However, Putin appeared defiant Friday in the face of a chorus of Western condemnation over what NATO says is clear evidence of Russian military aggression in Ukraine.

Moscow doesn’t want or intend to wade into any “large-scale conflicts,” Putin said at a youth forum, state-run Itar-Tass reported. A few breaths later, he made the point that Russia is “strengthening our nuclear deterrence forces and our armed forces,” making them more efficient and modernized.

“I want to remind you that Russia is one of the most powerful nuclear nations,” the President said. “This is a reality, not just words.”

His comments came as a British government source claimed that Russian troops had significantly ratcheted up their military incursion into Ukraine.

The source told CNN on Friday that Russia has moved 4,000 to 5,000 military personnel into Ukraine — a figure far higher than one U.S. official’s earlier claim of 1,000 troops.

The soldiers are aligned in “formed units” and fighting around the eastern cities of Luhansk and Donetsk, said the UK source. Some 20,000 more troops are on the border and “more may be on the way” to the border, the source added.

‘New Russian aggression’

The EU leaders meeting Saturday in Brussels may decide what action to take as soon as Saturday evening, EU foreign policy chief Catherine Ashton said, speaking after an informal meeting of foreign ministers in Milan, Italy.

Swedish Foreign Minister Carl Bildt, in Milan for the two-day talks, tweeted that they had been dominated by “new Russian aggression in Ukraine and consequences.”

“Very little doubt now that Russia will use also its regular armed forces to try to carve out a Novorossyia semi-state from Ukraine,” he said. “Novorossiya” is a Russian term meaning “New Russia” that has historical associations with the Russian Empire and has been embraced by the separatists.

German Chancellor Angela Merkel and U.S. President Barack Obama agreed in a phone call Thursday that they would have to consider tougher action against Russia.

The European Union and the United States have already slapped economic sanctions on targeted Russian individuals and businesses. The EU also has sanctioned certain sectors of the Russian economy, prompting Russia to retaliate with its own measures.

Russia is a major supplier of natural gas to European nations, complicating efforts to exert pressure on Moscow — especially as colder weather approaches.

Russia has repeatedly denied either supporting the rebels, or sending its own troops over the border. But its assertions have been roundly rejected by the West.

Troops dig trenches by Mariupol

A new front in the conflict opened up Wednesday around the southern coastal town of Novoazovsk, about 12 miles (20 kilometers) from the Russian border, with Kiev accusing Russian troops of seizing the town.

Col. Andriy Lysenko, spokesman for the Ukrainian National Defense and Security Service, told reporters Saturday that the Ukrainian military is working to fortify the port city of Mariupol, after Russian intelligence groups were observed there.

Many fear the city, which lies between Novoazovsk and Crimea, may be next in the rebels’ sights.

A CNN team in Mariupol saw Ukrainian forces strengthening defensive positions on the eastern outskirts of the city, reinforcing checkpoints and digging trenches along roads leading toward the Russian border.

Beyond these checkpoints, the team found a small advance detachment of Ukrainian troops on the main road about halfway between Mariupol and Novoazovsk. The soldiers said it had been quiet Saturday.

The CNN team encountered the first pro-Russian rebel roadblock a short distance outside Novoazovsk, with concrete blocks across the road and a heavy machine gun among the weaponry.

The Organization for Security and Cooperation in Europe has sent observers to Mariupol to monitor the situation.

Long queues formed at one Ukrainian checkpoint out of Mariupol on Saturday as people from rural areas tried to get in and out of the city.

Analysts suggest that Russia may have sent its forces into Novoazovsk to secure a land route from the border to the Crimean peninsula it annexed from Ukraine in March, and in order to throw Ukrainian forces making gains against besieged rebel forces in the cities of Donetsk and Luhansk off balance.

Fighting continued in the Luhansk area Saturday, Lysenko said.

The latest military escalation followed a meeting between Putin and Poroshenko in Belarus on Tuesday which had seemed to offer the prospect of diplomatic progress.

Denials ‘without credibility’

Russia annexed Ukraine’s Crimean peninsula in March, following the ouster of pro-Moscow President Viktor Yanukovych the previous month.

Violence broke out in the eastern Donetsk and Luhansk regions in April, as separatist leaders declared independence from the government in Kiev. Since mid-April, the conflict between the pro-Russia rebels and the Ukrainian military has cost more than 2,500 lives, according to the United Nations.

White House spokesman Josh Earnest said Friday that — whatever the Kremlin says — the reality is Russian troops are inside Ukraine and have fired on Ukrainian military positions.

NATO, which on Thursday produced what it says are satellite images showing Russian troops engaged in military operations inside Ukraine, also gave short shrift to Russia’s claims.

In response, Foreign Minister Sergey Lavrov said NATO had previously used “images from computer games” to — in his view — falsely make the case that Russian troops are in Ukraine, and said the “latest accusations are pretty similar.”

Russia accuses Ukraine of creating a humanitarian crisis in eastern Ukraine by launching a military operation to recapture the territory taken by the rebels. Russia has sent one aid convoy into the country without Ukraine’s permission and has said it intends to send more such convoys.

Barroso said there was an urgent need for international assistance but that it must be provided “in coordination with Ukrainian authorities and but also with respect to Ukraine’s sovereignty.”

Europe has already mobilized hundreds of millions in loans and grants for Ukraine, Barroso said, and over 1 billion euros more in loans could follow in the coming months.