Jake Sullivan, President Joe Biden's national security adviser, was coy regarding what military aid the United States would provide Ukraine and NATO should Russia cross the border.

"I will look you in the eye and tell you, as President Biden looked President Putin in the eye and told him today, that things we did not do in 2014, we are prepared to do now," Sullivan told reporters Tuesday.

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The U.S. would offer "defensive materiel to the Ukrainians above and beyond that which we are already providing," supplementing economic sanctions, Sullivan said.

"And we would fortify our NATO allies on the eastern flank with additional capabilities in response to such an escalation," he added.

Sullivan also defended the administration's position on the Nord Stream 2 pipeline, denying the widely held opinion that the project gifted Russia leverage and arguing it was beneficial to the U.S.

"The fact is that gas is not currently flowing through the Nord Stream 2 pipeline, which means that it's not operating, which means that it's not leverage for Putin," Sullivan said. "It is leverage for the West."

Biden was considering "all contingencies," Sullivan said amid reports the administration was looking at evacuation plans for U.S. staff and citizens.

"President Biden was direct and straightforward with President Putin, as he always is," he said. "There was a lot of give-and-take. There was no finger-wagging, but the president was crystal clear about where the United States stands on all of these issues."

Biden has already briefed European leaders on his two-hour call with Putin, according to Sullivan. He will speak with congressional leaders later Tuesday and Ukrainian President Volodymyr Zelensky on Thursday, Sullivan said.

Biden warned Putin that he and trans-Atlantic partners will roll out "strong economic and other measures in the event of military escalation" between Russia and Ukraine. Russian forces, including roughly 100,000 troops, have been gathering along the border as Ukraine clamors to be admitted into NATO. It is the largest surge of personnel and equipment since Russia's 2014 annexation of Crimea and the second of this year.

"President Biden reiterated his support for Ukraine’s sovereignty and territorial integrity and called for de-escalation and a return to diplomacy," a White House readout said.

A senior administration official had previewed that Biden and Putin would discuss Russia's military buildup on Ukraine's border, as well as cybersecurity and Iran nuclear deal negotiations.

The official insisted Biden did not seek conflict with Russia and that U.S. intelligence had not determined Putin would invade, but the source likened the military movement to what occurred prior to the Crimea annexation combined with a disinformation campaign.

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"In the event of an invasion, the need to reinforce the confidence and reassurance of our NATO allies and their eastern flank allies would be real, and the United States would be prepared to provide that kind of reassurance," the official said.