The White House on Monday warned that the "world needs to be prepared for a very long, difficult road ahead" as Russia continues its invasion of Ukraine for a twelfth day. 

White House press secretary Jen Psaki said the United States will continue to support the Ukrainians as they "fight back" but maintained that U.S. troops will not fight in the country and stopped short of vowing a ban on Russian oil imports.

More than 400 civilians in Ukraine have died since Russia’s invasion began 12 days ago, and another 800 have been injured, according to the Office of the United Nations High Commissioner for Human Rights. The United Nations also estimated Monday that 1,735,068 people overall have fled Ukraine since Feb. 24.

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"This is barbaric. It is horrific to watch. You have 1.5 million, if not more refugees crossing the border – you have mothers and children dead on the side of the road – this is heart-wrenching to watch," Psaki said during the press briefing Monday. "I think, for us who are working in public service, you watch in these moments, and there are limitations."

Ukraine Irpin

Ukrainians crowd under a destroyed bridge as they try to flee across the Irpin Rver on the outskirts of Kyiv, Ukraine, Saturday, March 5, 2022. (AP Photo/Emilio Morenatti)

"We are not going to send U.S. troops to fight in Ukraine against Russia," Psaki said. "The president is not going to do that – that is what you have to weigh as president of the United States."

Instead, Psaki said the Biden administration is providing $1 billion of expedited security assistance to Ukraine.

"I think any world leader would tell you that without the president’s leadership and without the United States rallying the world to hold Russia accountable, there would not be the kind of accountability and pressure on the financial system in Russia," Psaki said. "So we are doing everything that is in the interests of the United States, in the interests of our partners in NATO, to put pressure on President Putin, and to support the Ukrainians."

Ukraine-Russian war destroyed school building

A Ukrainian service member walks near a school building destroyed by shelling in Zhytomyr, Ukraine, March 4, 2022, as Russia's invasion continues.  (Reuters/Viacheslav Ratynskyi)

"We are very cognizant of this here, to be clear, that this is going to continue to be a very, very difficult period of time," Psaki said. "The Ukrainians are fighting bravely, we’re standing by them and supporting them, but Russia, you know, they are going to continue to fight, and they are going to continue to pursue what President Putin’s ambition is."

Psaki said the United States supports the "rights of the Ukrainian people to fight back," and touted Ukrainian President Volodymyr Zelenskyy for his "leadership" and actions "fighting bravely and courageously over the course of the last 12 days."

"I think it is also true that the world needs to be prepared for a very long, difficult road ahead while they are fighting bravely," Psaki warned. "And we are standing with them and supporting them."

Jen Psaki

Press secretary Jen Psaki speaks during a press briefing at the White House, Friday, March 4, 2022. (AP Photo/Carolyn Kaster)

But Psaki warned that the Russians "are still intending to grind out military advances in the short term just by sheer manpower and firepower."

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The White House’s comments come amid criticisms from Zelenskyy that the United States is not doing enough to aid Ukraine. Zelenskyy, on Monday, also reiterated his calls for a no-fly zone over Ukraine, pleading with Western allies: "How many deaths and losses are still needed to secure the sky over Ukraine?"

Ukraine’s calls for a no-fly zone come as the Biden administration and both Democratic and Republican lawmakers have made clear they are firmly against such a move.

Last week, the Biden administration ruled out setting up a no-fly zone for Russian aircraft over Ukraine, with senior defense officials saying enacting one would put the United States "in the fight" – as President Biden has maintained that U.S. military will not fight in Ukraine.

Zelenskyy, on Monday, also called for "boycotts" of Russian oil imports to the U.S. and European countries.

An official told Fox News Monday that the administration is considering a possible ban of oil imports from Russia to the United States, but said "no decision has been made at this time."

Ukraine President Volodymyr Zelenskyy

Ukraine President Volodymyr Zelenskyy is seen in a Zoom call with U.S. senators on Saturday, March 5, 2022. (Sen. Marco Rubio)

Russia is the third-largest producer of oil in the world, but the impact to U.S. oil imports would be "minimal" compared to allies, administration officials told Fox News.

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Russian oil exports account for about one-third of Europe’s oil imports. However, for the United States, Russian exports are just under 10% of U.S. overall imports.

But the calls to boycott Russian oil imports are not only coming from Zelenskyy – both Democrats and Republicans in Congress have also said it may be time for a ban on Russian imports.

Republicans and moderate Democrats alike are calling for the ramping up of American energy production, with GOP lawmakers criticizing the Biden administration for shutting down U.S. oil and gas pipelines.

As the war in Ukraine rages on, Americans are experiencing the highest gas prices since the 2008 financial crises, with the national average reaching more than $4 per gallon. 

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When asked about potential congressional legislation to ban Russian oil imports to the U.S., and whether the president would sign such legislation into law, Psaki quipped: "Is a bill on its way over here that’s passed Congress? I don’t think so." 

"The president has not made a decision at this point in time," she said. 

Vladimir Putin

Russian President Vladimir Putin has signed a law that could punish journalists with up to 15 years in prison for reporting so-called "fake" news about his military invasion of Ukraine. (Yuri Kochetkov/Pool)

Meanwhile, President Biden held a secure video call with French President Emmanuel Macron, German Chancellor Olaf Scholz and British Prime Minister Boris Johnson, during which the White House said the leaders "affirmed their determination to continue raising the costs on Russia for its unprovoked and unjustified invasion of Ukraine."

Psaki was asked whether the White House is concerned that the economic sanctions from the West, and Western companies suspending operations in Moscow, could strengthen ties between Russia and China.

"We know they were moving closer long before the invasion, which is, I think, an important component here," she said. "As it relates to the economic ties, there is not a way, just based on purely their place in the economic marketplace, for China to completely backfill the impact of the G-7 and others in the world and the financial sanctions."