"Recognition of Donetsk & Luhansk separatist ‘republics’ by Russia is an intolerable violation of international law," Nauseda tweeted. "It also means unilateral withdrawal from Minsk agreements."
"Lithuania's support to Ukraine's sovereignty and territorial integrity is ironclad," he added.
Late on Monday, Nauseda discussed the EU's response to the worsening security situation with European Council President Charles Michel, the Lithuanian president's office said in a press release.
Nauseda underlined the need for an appropriate response from the EU to Russia's actions. The two officials also discussed a possible package of sanctions against Russia in response to its illegal actions.
"Ignoring international law and disrespecting the territorial integrity of a sovereign country is completely intolerable," Nauseda said. "The international community must respond immediately to these actions by Russia and not allow such behavior to become the new norm."
The Western world must stand firmly behind Ukraine in this situation and take decisive action to prevent further escalation, according to the president.
Other Lithuanian politicians and Western leaders have also condemned Russia's decision, and EU leaders promise a strong, firm and united response.
"In view of Russia's illegal decision, just announced by Putin, I will initiate a resolution in the Seimas stating that any change to the status of Donbass region as an integral part of Ukraine will NEVER be recognized," Viktorija Cmilyte-Nielsen, the speaker of the Lithuanian parliament, has tweeted.
"Such an unlawful act must be met with strongest possible response," she said.
The West had repeatedly warned Russian not to recognize the separatists, as such a move would effectively bury the fragile peace process in Donbas.