Russian President Vladimir Putin has made repeated threats in recent weeks about deploying nuclear weapons if his country is attacked.

Putin said he sees Russia's war with Ukraine as an ideological battle between Russia and the West. By last week, Russia had gained over 18% of Ukraine's territory, including four regions annexed on Friday.

The U.S. called the annexations a violation of international law and increased sanctions on hundreds of people and companies connected to Russia.

The land gains could pave the way for nuclear action under the country's nuclear doctrine, which permits a nuclear strike after "aggression against the Russian Federation with conventional weapons when the very existence of the state is threatened."

President Biden condemned the annexation last week, saying "these actions have no legitimacy." In a phone call Tuesday with Ukrainian counterpart Volodymyr Zelenskyy, Biden reiterated the United States' support for Ukraine.

"President Biden also affirmed the continued readiness of the United States to impose severe costs on any individual, entity, or country that provides support to Russia's purported annexation," a White House press release said.

The Biden administration also pledged an additional $625 million in military, security and technology aid to Ukraine.

While Putin has maintained his stance about the severity of nuclear threat, leaders in the west remain skeptical.

"I would advise to remain calm," Mark Rutte, Dutch Prime Minister said last month, adding that the region's leaders were familiar with Putin's rhetoric.

Alexander de Croo, Belgium's Prime Minister, had a similar position.

"We must not add fuel to the fire," he said. "We must be clear in our position and continue to support Ukraine."

The Biden administration and U.S. officials likewise are wary of Putin's nuclear threats.

"We certainly have seen nothing that would give us pause to change our own strategic deterrent posture or ability to defend our allies and partners on the European continent," John Kirby, the National Security Council's Strategic Communications Coordinator told MSNBC on Tuesday.