On Sunday, a few dozen elite American troops and military hardware were spotted landing in southeastern Poland near the Ukraine border, following U.S. President Joe Biden's order to deploy 1,700 troops in response to fears of a Russian invasion of Ukraine.

Russian President Vladimir Putin may order an attack on Ukraine "within days or weeks," White House national security adviser Jake Sullivan warned Sunday, as the U.S. and its European allies stepped up efforts to offer Putin a diplomatic solution to the crisis.

On Thursday, Biden ordered nearly 3,000 additional troops to Poland and Romania, as Washington seeks to reassure nervous NATO allies. According to the Pentagon, approximately 1,800 service members, primarily from the 82nd Airborne Division, would deploy to Poland from Fort Bragg, North Carolina.

As this unfolded, thousands of people took to the streets of Ukraine's second largest city, carrying banners proclaiming "Kharkiv is Ukraine" and "stop Russian aggression," as the country braced itself for a possible Russian invasion.

Weeks of diplomacy between the West and Russia have yielded no results following Russia's amassing of tens of thousands of troops near Ukraine's borders.

The U.S. accused the Kremlin on Thursday of a sophisticated plot to fabricate an attack by Ukrainian forces in order to use it as a pretext for military action against its neighbor.

According to Pentagon spokesman John Kirby, the scheme included the creation of a graphic propaganda video featuring staged explosions and the use of dead bodies and actors portraying bereaved mourners. The U.S. has not provided detailed evidence to support its claims.

A C17 aircraft arrived on Sunday, following a plane carrying the 82nd Airborne Division's commanding general, U.S. Army Maj. Gen. Christopher Donahue, who landed at Rzeszow-Jasionka airport on Saturday, as well as a few aircraft loaded with U.S. military equipment and an "advance group."

"Our national contribution here in Poland demonstrates our solidarity with all of our European allies, and clearly, in this time of uncertainty, we know that we are stronger together," Donahue said on Sunday.

Poland's defence minister, Mariusz Blaszczak, stated shortly after the plane touched down that this was the first group of American soldiers "from an elite unit."

According to Blaszczak, additional planes will land in the coming hours, and soldiers will conduct operations in southeastern Poland.

NATO members responding collectively is "the best response to a threat, the only way to ensure Poland's security and that of other NATO members on the alliance's eastern flank," Blaszczak added.