(WASHINGTON) -- A senior U.S. defense official said on Friday the Russian offensive is going much slower than planned in part because of the strength of the Ukrainian resistance.

“We also assess that because of this slow and uneven progress, again, without perfect knowledge of every aspect of the Russian plan, we do believe and assess that they are behind schedule in what they were trying to accomplish in the Donbas,” the official said, speaking on condition of anonymity to discuss the U.S. military’s assessment.

He said the U.S. believes the Russians are “at least several days behind where they wanted to be” as they try to encircle Ukrainian troops in the east.

As the troops try to move north out of Mariupol so they can advance on Ukrainian forces from the south, their progress has been “slow and uneven, and certainly not decisive in any, in any event,” the official said.

The $40 billion U.S. package of assistance for Ukraine as it tries to fend off Russia's aggression is hitching a ride on a commercial flight to South Korea so it can be signed by President Joe Biden.

The Senate voted Thursday to finalize new military and humanitarian assistance to Ukraine as Biden was making his way to the South Korean capital. A White House official says the bill is being flown to Seoul by a U.S. government official who was already planning to travel to the region on a commercial flight.

As the Associated Press reported, bills that have needed an urgent signature have been routinely flown by White House aides for decades to the location of the sitting president, if they were abroad.

Then-President George W. Bush flew back to Washington from his Texas ranch to in 2005 to sign legislation that would require doctors to continue feeding a comatose Florida woman, Terri Schiavo. Her husband wanted to let her die.

The Kremlin-backed leader of Russia’s southern province of Chechnya has posted a video in which he warns that Poland could be next after Ukraine.

Ramzan Kadyrov, who is famous for his bluster, said in the video he posted to his official Telegram page that Ukraine was “a done deal” and that “if an order is given after Ukraine, we’ll show you [Poland] what you’re made of in six seconds.”

Poland, which borders Ukraine, has provided its neighbor with weapons and other aid since Russia invaded Ukraine on Feb. 24. It has also welcomed in millions of Ukrainian refugees.

Kadyrov later urged Ukrainian President Volodymyr Zelenskyy to “finally come to his senses and accept the conditions offered by our president [Vladimir Putin].”

Kadyrov has repeatedly used social media to boast about Chechen fighters’ alleged performance against Ukrainian troops and to make other unconfirmed statements about the war in Ukraine.