"The situation demands consideration and the president will likely take a decision on calling a state of emergency, which would require summoning the Seimas," she told the public broadcaster LRT.
President Gitanas Nauseda is convening a meeting of the State Defense Council at 9 a.m.
Laurynas Kasciunas, chairman of the parliamentary Committee on National Security and Defense, said earlier on Thursday that the council might also decide on consultations with NATO and on increasing Lithuania's combat readiness.
According to Simonyte, Russia has effectively declared war on Ukraine.
"Despite what Mr. Putin says in what seems to be a pre-recorded address message about some special operation in Donbass, I think what is actually happening can be called a full-scale military invasion," she told LRT.
In the prime minister's words, it is now important for the West to maintain the unity it has shown since the Kremlin started issuing ultimatums.
"We have to hold talks on diplomatic pressure, political pressure, economic pressure on the Kremlin regime, but also support for Ukraine," she said.
In the early hours of Thursday, Russian President Vladimir Putin announced a "special military operation" in Ukraine to defend pro-Kremlin separatists and "demilitarize and denazify" its western neighbor.
Shortly after he spoke, explosions were heard in Kyiv and several other cities.