Russian Foreign Minister Sergeĭ Lavrov warned that if NATO sends peacekeepers into Ukraine amid Russia's invasion, that will lead to a direct military clash between the Russian forces and the alliance.

"Sending NATO peacekeepers to Ukraine would lead to a direct clash between the Armed Forces of the Russian Federation and the alliance," Lavrov said Wednesday morning, according to the Russian state-news agency TASS

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U.S. Ambassador to the United Nations Linda Thomas-Greenfield on Sunday ruled out the possibility of the U.S. taking part in peacekeeping operations inside Ukraine, stating that the Biden administration will not send U.S. troops to the country.

Russian Foreign Minister Sergey Lavrov

Russian Foreign Minister Sergey Lavrov pauses during a news conference in Moscow on Nov. 30, 2021. (Russian Foreign Ministry Press Service via AP)

"The president has been very clear that we will not put American troops on the ground in Ukraine," Thomas-Greenfield said. "We don't want to escalate this into a war with the United States."

"But we will support our NATO allies," she added. Biden "has made clear that if there is an attack on any of our NATO allies, under Article 5, that we will support those countries and defend those countries." 

Linda Thomas-Greenfield

U.N. ambassador nominee Linda Thomas-Greenfield testifies during for her confirmation hearing on Capitol Hill on Jan. 27, 2021. (Greg Nash/Pool via AP)

The Polish outlet Onet.Pl reportedly gained access to a secret project at Poland's Ministry of National Defense, aiming to deploy peacekeepers from a number of NATO countries to Ukraine. Warsaw is expected to officially present it at the NATO summit on March 24. The U.S. will not participate but may agree to a mission involving some other countries. 

Ukraine Cars night air raid in the village of Bushiv

Gutted cars following a night air raid in the village of Bushiv, 40 kilometers west of Kyiv, Ukraine, Friday, March 4, 2022. (AP Photo/Efrem Lukatsky)

Lavrov's warning suggests that Russia would consider the presence of NATO peacekeepers a military escalation and a cause of war against the alliance. 

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The war has dragged on for about a month as Russia's superior troop numbers have run into unexpectedly fierce Ukrainian resistance. The battle for the southern port city of Mariupol has grown increasingly fierce, while cities like Ukraine's capital, Kyiv, and its second city, Kharkiv, are effectively under siege. Russia has also targeted cities in the West of the country, not far from the border with Poland, where more than 2 million of Ukraine's 3 million refugees have reportedly fled.