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A bipartisan group of U.S. senators traveled to Ukraine to meet with President Volodymyr Zelenskyy and other officials on a trip to show solidarity amid fears of Russian aggression.

"We believe that this is a crucial time for us to come," Sen. Rob Portman, R-Ohio, a member of the Foreign Relations Committee, told reporters between meetings. "To Russia: We stand with the Ukrainian people and with this government. In fact, Congress recently passed an increase to $300 million in security funding to Ukraine in the National Defense Authorization Act."

With tens of thousands of Russian troops massed on Ukraine’s border, the Biden administration is threatening unprecedented sanctions and other tough steps if Russia takes military action against Ukraine. The administration has also prepared a new package of military aid for Ukraine, in addition to the U.S. military assistance that is already flowing to Kyiv, current and former officials said.

The congressional delegation also included Democratic Sens. Chris Murphy and Richard Blumenthal of Connecticut, Jeanne Shaheen of New Hampshire and Amy Klobuchar of Minnesota and Republican Sens. Kevin Cramer of North Dakota and Roger Wicker of Mississippi.

"I think Vladimir Putin has made the biggest mistake of his career in underestimating how courageously the people of Ukraine will fight him if he invades," Blumenthal, a member of the Armed Services Committee, told reporters later Monday after the meeting with Zelenskyy. "And we will impose crippling economic sanctions, but more important, we will give the people of Ukraine the arms, lethal arms, they need to defend their lives and livelihood."

Ukraine was hit with a massive cyberattack last week that downed more than 70 government websites. National security adviser Jake Sullivan told CBS News on Sunday that the U.S. still working to determine who was behind the attack but that "this is part of the Russian playbook."

"If it turns out that Russia is pummeling Ukraine with cyberattacks, and if that continues over the period ahead, we will work with our allies on the appropriate response," he said.

Cramer, a member of the Armed Services Committee, said Monday that President Joe Biden is "right to not wait for congressional authorization" as he moves to "build a set of multilateral sanctions" and that additional action from Capitol Hill may be coming.

"My belief is that we will be able to come together in Congress around the sanctions authority to make sure that President Biden has everything he needs and is coordinated with our allies to bring the crushing set of sanctions before any invasion plans," Cramer said.

Wicker, also a member of the Armed Service Committee, suggested using the Magnitsky Act to sanction "actual individuals who are violating international law."

"There is no question the aggression has already begun," Wicker said.

Speaking to reporters later Monday, Murphy said that the senators on the trip felt an urgency to display bipartisan unity in support of Ukraine after the Senate voted against imposing sanctions over Nord Stream 2 pipeline last week.

“I think it adds to the deterrent effect for Putin to see Republicans and Democrats messaging together in Kyiv, rather than watching Republicans and Democrats split on the question of sanctions, as we did last week," he said. "So, you know, we're trying to do our small part to telegraph to Putin and his government that last week's vote is not a precursor to how we will act moving forward to try to put together a set of deterrent measures.”