Workers lower a coffin containing the body of Alexander Demyanenko, who was killed in Saturday’s shelling of Mariupol, Ukraine, during burial services for victims on Jan. 26. (Evgeniy Maloletka/AP)

As Western leaders mulled new punitive measures against Russia over its involvement in the latest violence in eastern Ukraine, Russian leaders lashed out Monday, accusing the West of using the events to incite anti-Russian hatred and of playing an on-the-ground role in furthering the fighting.

The sharpened charges punctuated an increasingly tense standoff between Russia and the West as the two sides exchange accusations over who bears ultimate responsibility for the worsening situation in eastern Ukraine, a conflict that both Russian and Western leaders have accused their counterparts of manipulating for the sake of greater global ambitions.

“Who is really out there fighting?” Russian President Vladimir Putin said to a group of students in St. Petersburg on Monday, when asked about Ukraine. “It is not even the army — it’s a foreign legion, in this case, a NATO foreign legion.”

The “NATO foreign legion,” Putin asserted, is operating with the goal of “geopolitical containment of Russia, which absolutely does not coincide with the national interests of the Ukrainian people.”

NATO Secretary General Jens Stoltenberg bluntly dismissed Putin’s charge Monday as “ridiculous.”

“The foreign forces in Ukraine are Russian,” Stoltenberg said. “That is, in a way, the problem: that there are Russian forces in Ukraine and Russia backs the separatists.”

As Brussels and Moscow escalate their war of words, clashes between Ukrainian troops and pro-Russian rebels are worsening.

Pro-Russian rebels from Donetsk announced a new offensive Monday against Ukrainian troops in Debaltseve, a city along a road connecting the separatist strongholds of Donetsk and Luhansk that has been nearly encircled by pro-Russian militias. A spokesman for the Ukrainian military said Sunday that the bombardment of the city has resulted in civilian casualties.

A rocket attack Saturday on Mariupol caused even more civilian casualties, killing 30 and wounding more than 100, in one of the conflict’s deadliest incidents.

Kiev and its Western allies — citing evidence from observers with the Organization for Security and Cooperation in Europe — alleged that the rockets had been fired from territory under the control of pro-Russian rebels. They pointed a finger at Russia for, as U.S. Secretary of State John F. Kerry put it, “aiding and abetting” such attacks by sending weapons across the border and directing the separatists through “operational command and control.”

Russia has continued to put the blame on Kiev and its supporters.

“We know how cynically such tragedies are used to distort the truth,” Russian Foreign Minister Sergei Lavrov said Monday, suggesting that the West was using the attack in Mariupol to promote “anti-Russia hysteria” and give those who are out for “blood” an excuse to impose new sanctions on Russia.

“I hope that reason and objective, legal national interests will still prevail,” he said.

Western leaders have not specified what further measures against Russia they are considering, though President Obama said Sunday that he would consider all options short of military action. Obama and NATO’s Stoltenberg defended current economic sanctions for demonstrating to Russia that its policy in Ukraine comes at a cost.

But thus far, sanctions do not appear to have persuaded Moscow to change course on Ukraine.

In the meantime, Russia’s financial troubles are being exacerbated by other forces.

On Monday, Standard & Poor’s downgraded Russia’s credit rating to junk status, a move that was expected but is nonetheless the bleakest assessment of Russia’s creditworthiness in more than a decade. The downgrade could accelerate already record outflows of foreign capital and the weakening of the ruble, which lost more than 50 percent of its value during the past year.