The momentum of Russian forces in Ukraine appears to have been slowed by fuel and logistics shortages and by "stiff resistance," according to a U.S. senior defense official.

The defense official said Russian troops appear to be having fuel and logistics shortages near Kharkiv in northeast Ukraine and with units advancing from the north to Kyiv, the capital city, but also credited the slow down of the Russian invasion to the resistance.

The official said they believe the advance was slowed in part "by resistance by the Ukrainians who have been quite creative in finding ways to attack columns."

The official said the Russians have still not taken any cities. The main Russian advance forces designated for Kyiv are roughly 19 miles from the city center, according to the official, who added that there is fighting inside the city center due to Russian reconnaissance units there.

"We're certainly not disputing that there's fighting going on in Kyiv, but it is at a fairly low level," the official said.

The officials said there is "no reason to doubt" reports of Russian reconnaissance units wearing Ukrainian uniforms to try to disguise themselves and what they're doing.

Russia has launched 320 missiles against Ukraine since the invasion began last week, and its troops have adopted alarming "siege tactics" around the northern Ukraine city of Chernihiv.

"They've had trouble around Chernihiv, and it appears that they are adopting a siege mentality, which any student of military tactics and strategy in history will tell you when you adopt siege tactics, it increases the likelihood of collateral damage to civilian infrastructure as well as to civilian life because a siege basically becomes an all-out effort to take a city without regard to civilian infrastructure," the official said. "So that's worrying and that's concerning. And we're seeing the beginnings of that sort of tactical approach by the Russians."

The official said Russia has committed two-thirds of its combat power designated to the Ukraine invasion.

"They have a third of it that has not been committed," the official said. "They have a significant amount of combined arms capabilities still at their at their beck and call."

The European Union announced major moves against the Russian government and businesses over the country's invasion.

In a first, the EU will finance the purchase and delivery of weapons and other equipment to a country that is under attack, officials said.

The EU will also shut down its airspace for "Russian-owned, Russian registered or Russian-controlled aircraft."

"They won't be able to land in, take off or overfly the territory of the EU," Ursula Gertrud von der Leyen, the president of the European Commission, announced.

In addition, the EU said it would ban the RT and Sputnik news agencies and would develop "tools to ban their toxic and harmful disinformation in Europe."

The EU also announced it would be stopping exports of products to Belarus, including mineral fuels, tobacco, wood and timber, cement, iron and steel. It will also sanction Belarusians who are supporting the Russian war effort.

Von der Leyen also said the EU will "welcome with open arms those Ukrainians who have to flee from Putin's bombs" and is encouraging efforts to support refugees.

"President Zelenskyy's leadership and his bravery and the resilience of the Ukrainian people are outstanding and impressive," von der Leyen said. "They are an inspiration to us all."

-ABC News' Christine Theodorou

Three people were killed, including a child, and one child was injured after cluster munitions hit a preschool in northeastern Ukraine Friday, Amnesty International reported.

Civilians were taking shelter inside the Sonechko nursery and kindergarten in Okhtyrka in Sumy Oblast when the munitions were dropped, according to the NGO.

Amnesty International says the attack appears to have been carried out by nearby Russian forces.

-ABC News' Christine Theodorou

Japan announced it is joining the U.S. and other European countries to disconnect selected Russian banks from the SWIFT system and sanction President Vladimir Putin and other Russian leaders.

Japan's announcement means the entire G-7 supports removing Russia from the crucial messaging system used by large banks around the world.

White House press secretary Jen Psaki released a statement Sunday praising Japanese officials for their decision.

"Prime Minister Kishida and the government of Japan have been leaders in condemning President Putin’s attack on Ukraine and we will continue working closely together to impose further severe costs and make Putin’s war of choice a strategic failure," she said in a statement.

-ABC News' Ben Gittleson

Secretary of State Antony Blinken closed his remarks to the U.N. Security Council meeting by challenging the Russian Federation to "announce today -- with no qualification of equivocation or deflection -- that Russia will not invade Ukraine, stated clearly stated plainly, to the world."

"And then demonstrate it by sending your troops, your tanks, your planes back to their various can hangars and sending your diplomats to the negotiating table," he added.

Blinken laid out how the U.S. believes Russia will attack Ukraine -- but said he would welcome being wrong and for Russia to withdraw.

"Now, I'm mindful that some have called into question our information, recalling previous instances where intelligence ultimately did not bear out," he said, apparently referring to a similar address then-Secretary of State Colin Powell famously made to the Security Council presenting U.S. intelligence to justify the Iraq War. "But let me be clear, I am here today not to start a war, but to prevent one," he said, citing allies that agree with U.S. assessments.

"If Russia doesn't invade Ukraine, then we will be relieved that Russia changed course and proved our predictions wrong. That would be a far better outcome in the course we're currently on. And we'll gladly accept any criticism that anyone directs at us," Blinken said.

He continued, "Russia can still make if there's any truth to his claim that is committed to diplomacy. Diplomacy is the only responsible way to resolve this crisis"

Blinken also said that he sent a letter to Russia's Foreign Minister Sergei Lavrov earlier Thursday proposing that they meet next week in Europe following their talks in recent weeks "to discuss the steps that we can take to resolve this crisis without conflict" and that U.S. is also proposing meetings at the NATO Russia Council and the OSC Permanent Council.

"These meetings can pave the way for a summit with key leaders in the context of de-escalation to reach understandings on our mutual security concerns," he added.