(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.

Ukraine’s ambassador-designate to Canada says Russia must be held accountable for its troops committing sex crimes, including against children.

Yulia Kovaliv told a Canadian House of Commons committee on Monday that Russia is using sexual violence as a weapon of war and said rape and sexual assault must be investigated as war crimes.

She said Russia also has kidnapped Ukrainian children and taken them to Russian-occupied territories and now Russia itself. Ukraine is working with partners to find the children and bring them back.

“Russians, a few days ago, killed a young mother and taped her living child to her body and attached a mine between them,″ the ambassador said. She said the mine detonated.

All of Russian society, and not just President Vladimir Putin “and his proxies," should bear responsibility for the war on Ukraine because more than 70% of Russians support the invasion, Kovaliv said.