Obama administration voices concern, considers further sanctions.
State Department spokeswoman Jen Psaki says Moscow is engaged in a "pattern of escalating aggression in Ukraine," now including combat forces, armored vehicles, artillery and surface-to-air missiles.
Psaki says the U.S. could increase economic sanctions against Russia. But she says no such decisions have been made. And to help Ukraine, she says Washington is focused on nonlethal forms of assistance and not military equipment.
The administration says Russia has suffered severe economic costs for its conduct in Ukraine, including the March annexation of the Crimean Peninsula.
However, all the American pressure has done little to persuade President Vladimir Putin to end Russia’s intervention.
Officials say heavily-armed Russian-backed separatists have captured new territory in southeastern Ukraine, far from their previous battles with government troops. The battles in and around the town of Novoazovsk have added to Western concerns that Russia is supporting the opening of a new front in Ukraine.
Heavy shelling accompanied rebel fighters with dozens of tanks and armoured vehicles who entered Novoazovsk, a strategic resort town of 40,000 people on the Azov Sea.
The new southeastern front has raised fears the separatists are seeking to create a land link between Russia and Crimea.
Witnesses said the offensive with heavy weapons added to growing evidence that the rebels receive support from neighboring Russia.
To travel to Novoazovsk, pro-Russian separatists would have to travel through government controlled areas. The region is easily accessible from the nearby Russian border.
A pro-Russian rebel leader in eastern Ukraine admitted that as many as 4,000 Russian citizens are fighting in their ranks.
Alexander Zakharchenko said they included former service people or current personnel on leave.
He was speaking as the rebels threatened to take the key port of Mariupol, after opening the new front in the southeast.
Poland’s Prime Minister Donald Tusk told parliament that his nation had also received independent confirmation that the Russian military was active in Ukraine.
"We have received information, which has been confirmed in the last few hours, that Russian troops are active in eastern Ukraine," Tusk said.
"The information is from the NATO military alliance and confirmed by our intelligence, and is basically unambiguous," he told legislators.
The United States believes Russia is helping rebel forces expand that fight with tanks, rocket launchers and armoured vehicles. "These incursions indicate a Russian-directed counteroffensive is likely underway in Donetsk and Luhansk," State Department spokeswoman Psaki told reporters, referring to two rebel regions in eastern Ukraine.
"Clearly that is of deep concern to us."
She also said the U.S. administration is "concerned about the Russian government’s unwillingness to tell the truth even as its soldiers are found 30 miles inside Ukraine.”
Kiev has accused Russia of launching a new incursion across its border. Ukraine said Wednesday it captured another Russian serviceman after releasing a video of 10 other seized Russian soldiers who reportedly entered the country illegally. Moscow has denied it actively supports the rebels.
Kiev also said that fighting in the towns of Horlivka and Ilovaysk to the north and east respectively had killed some 200 pro-Russian separatists and 13 Ukrainian troops in recent days.
Additionally, several residents were killed or injured as shelling hit the rebel-held stronghold of Donestk, where the cultural center caught fire and a residential building was seriously damaged.
More than 2,000 people have been killed in months of fighting, including 298 people, when a Malaysian airliner was shot down last month, allegedly by pro-Russian rebels.
Vatican Radio contributed to this report.
Since you are here…
…we’d like to have one more word with you. We are excited to report that Aleteia’s readership is growing at a rapid rate, world-wide! Our team proves its mission every day by providing high-quality content that informs and inspires a Christian life. But quality journalism has a cost and it’s more than ads can cover. We want our articles to be accessible to everyone, free of charge, but we need your help. To continue our efforts to nourish and inspire our Catholic family, your support is invaluable. Become an Aleteia Patron today for as little as $3 a month. May we count on you?