A Ukrainian national guard serviceman stands atop a destroyed Russian tank in an area near the border with Russia, in the Kharkiv region, Ukraine, September 19, 2022. [AP Photo]

European Union foreign ministers have agreed to bolster their military support for Ukraine and study a new package of sanctions targeting Russian individuals and certain sectors of the Russian economy.

EU foreign policy chief Josep Borrell told reporters late Wednesday after convening a special ministerial meeting in New York that the details of the sanctions package still need to be determined by EU officials, but that he is sure there will be unanimous support.

Borrell said it was important for the ministers to meet and send a "powerful message" on the same day that Russian President Vladimir Putin announced the partial mobilization of his country's military reserves, a call-up of 300,000 fighters. Borrell said Putin was "trying to destroy Ukraine."

Kremlin spokesman Dmitry Peskov said Thursday that the mobilization was necessary because Russia was "de facto facing all of NATO," the U.S.-led Western military alliance that has shipped billions of dollars of armaments to Ukraine to help fend off Russia's invasion of Ukraine

The EU foreign ministers' meeting came as Russia and Ukraine agreed to what appeared to be their largest prisoner swap since the war began in late February, with 215 Ukrainians freed from Russian captivity, 55 Russians and pro-Russia Ukrainians sent to Russia and 10 foreign nationals released, including two U.S. military veterans who had fought alongside Ukrainian soldiers.

Despite the prisoner exchange, fighting raged on, with at least six people killed Thursday, one in the southern city of Zaporizhzhia and the five others in the Russian separatist-controlled city of Donetsk.

Pro-Moscow authorities in Ukraine's Russian-controlled regions are preparing to hold referendums starting Friday on becoming part of Russia, which then could allow Moscow to escalate the war in defense of the annexed territory. The votes are planned in the Luhansk, Kherson, Zaporizhzhia and Donetsk regions, although Ukraine and its Western allies have called them sham referendums and have no plans to recognize the outcome as legitimate.

Borrell said that in addition to "the immense suffering brought by the Russian aggression upon the Ukrainian people, Russia has chosen to further extend the cost of war also for their own Russian population."

He said Putin's apparent reference to Russia's willingness to use nuclear weapons if necessary to protect itself represented "an irresponsible and cynical attempt to undermine our steadfast support to Ukraine."

"These threats jeopardize in an unprecedented scale international peace and security," Borrell said. "But they will not shake our determination. They will not shake our resolve, our unity to stand by Ukraine and our comprehensive support to Ukraine's ability to defend its territorial integrity and sovereignty as long as it takes."

Putin said in a televised address Wednesday the mobilization of reserves, which followed Ukrainian gains in a counteroffensive in northeastern Ukraine, is necessary to protect Russia's homeland and sovereignty.

Putin said the West is trying to weaken and destroy Russia, and that his country will "use all the means at our disposal to protect Russia and our people."

"In its aggressive anti-Russian policy, the West has crossed every line," he said. "This is not a bluff. And those who try to blackmail us with nuclear weapons should know that the weathervane can turn and point towards them."

Britain's defense ministry said Thursday that Russia's mobilization "is likely to be highly unpopular with parts of the Russian population" and that Putin is taking "considerable political risk in the hope of generating much needed combat power."

Street protests against the mobilization erupted in Moscow and other Russian cities, with police arresting 1,300 demonstrators.

Prisoner exchange

Ukrainian President Volodymyr Zelenskyy said Thursday that most of the freed Ukrainian fighters in the prisoner swap had defended a steel plant during Russia's siege of Mariupol early in the conflict.

Zelenskyy also said five of the 10 foreign nationals released were from Britain, with the others from Croatia, Morocco and Sweden, along with the two former U.S. service members.

The most prominent of the 55 Russians and pro-Russia Ukrainians sent to Russia was Viktor Medvedchuk, a Putin ally who had escaped house arrest days before Russia's invasion only to be recaptured in April.

Some information for this story came from The Associated Press, Agence France-Presse and Reuters.