WASHINGTON — President Biden on Tuesday announced harsh new sanctions aimed at punishing Russia for what he called “the beginning of a Russian invasion of Ukraine,” joining European leaders in imposing severe economic consequences for blatant violations of national sovereignty.

Speaking from the East Room of the White House, Mr. Biden condemned President Vladimir V. Putin of Russia for his aggression against Ukraine, saying that the Russian action is “a flagrant violation of international law and demands a firm response from the international community.”

Mr. Biden said the United States is imposing “full blocking” on two large Russian financial institutions and “comprehensive sanctions” on Russian debt.

“That means we’ve cut off Russia’s government from Western finance,” he said. “It can no longer raise money from the West and cannot trade in its new debt on our markets or European markets either.”

He also said that the United States would impose sanctions on Russian elites and their families, an effort to ensure that those closest to Mr. Putin do not escape financial pain that is expected to hit hard for average Russian citizens.

Mr. Biden delivered his speech less than a day after Mr. Putin issued a decree sending troops into two separatist-held regions in Ukraine. Russia’s Parliament authorized the use of military force abroad on Tuesday, taking another step toward what Western officials fear could be an full-scale assault against Ukraine.

Western leaders said Russian troops have already entered Ukraine in what the president labeled an “invasion.”

But as the tough global response takes shape, Mr. Biden and his counterparts have struggled to balance the need to take swift and severe action while preserving the possibility of even greater sanctions on Russia if Mr. Putin escalates the conflict by attempting to seize the entire country — a war that could kill tens of thousands of people.

Mr. Biden said that he would ratchet up sanctions against Mr. Putin if Russia went further than it had already.

“Russia has now undeniably moved against Ukraine,” he said. “Today, I’m announcing the first tranche of sanctions to impose costs on Russia.”

He added: “We’ll continue to escalate sanctions if Russia escalates.”

Germany announced Tuesday that it would halt certification of a natural gas pipeline linking it with Russia. The British government said that it would sanction members of the Russian Parliament who voted to recognize the independence of the separatist areas and would create legislation to ensure that no British individual or company could do business with the regions, Donetsk and Luhansk.

“This is the first tranche, the first barrage of what we are prepared to do,” Prime Minister Boris Johnson told Parliament on Tuesday.

In the United States, Jon Finer, Mr. Biden’s deputy national security adviser, said that Russia’s forces had begun to move into Ukraine, declaring on CNN that “an invasion is an invasion, and that is what is underway.” But he and Mr. Johnson shared the same sentiment, saying that “we’ve always envisioned waves of sanctions that would unfold over time in response to steps Russia actually takes.”

Two European officials said Tuesday that Russia had sent troops into the area, but Russia’s Foreign Ministry denied having done so.

Mr. Putin remained defiant in the face of the worldwide condemnations of his decision to recognize the independence of the so-called Donetsk and Luhansk People’s Republics created after Russia fomented a separatist war in eastern Ukraine in 2014.

In what sounded like an ultimatum, he demanded that Ukraine recognize Russia’s claim to Crimea and relinquish its advanced weapons.

A deputy defense minister, Nikolai Pankov, said that Ukraine had gathered 60,000 troops to attack the Russia-backed separatist enclaves in the country’s east — a step that Ukraine denies having any plans to take.

“Negotiations have reached a dead end,” Mr. Pankov said in a televised speech. “The Ukrainian leadership has taken the path of violence and bloodshed.”

There was no immediate sign of major military escalation in eastern Ukraine, but fearful Ukrainians boarded buses out of the separatist areas as Ukraine’s president, Volodymyr Zelensky, urged his beleaguered nation to “keep a cool head” in the crisis.

Mr. Zelensky insisted that Ukraine would not yield territory, and his defense minister, Oleksiy Reznikov, appeared to be girding his country’s troops for battle.

“Ahead will be a difficult trial,” Mr. Reznikov said in a somber message released by the military. “There will be losses. You will have to go through pain and overcome fear and despondency.”

A day earlier, Mr. Putin delivered a long, fiery speech that described Ukraine as part of Russia, calling the government in Kyiv little more than a “puppet” of the United States and its leaders solely responsible for whatever “bloodshed” may come next.

“As for those who captured and are holding on to power in Kyiv,” he said, referring to the Ukrainian capital, “we demand that they immediately cease military action.”