Russia-Ukraine updates: Russian missiles hit close to nuclear reactors: IAEA director

The number of Russian casualties from the war is "very significant," a senior Pentagon official said Friday.

"And when you look at the numbers of people that they have lost, [it's] impossible to believe that they're not having to figure out ways to replace them if they want to continue to go toe-to-toe with Ukrainians," the official said.

Five months into the war, a senior military official called the invasion "a gigantic strategic failure on the side of the Russians."

"They had to reposition their people out from the north around to the east to try and make some gains. And those gains, as you've seen, have been relatively small -- at a gigantic cost. So I'd really hesitate to give you an idea how long this lasts," the official said. "It certainly looks like it's going to last as [long as] the Ukrainians are able to go toe-to-toe."

-ABC News’ Luis Martinez

Secretary of State Antony Blinken said he spoke with Russian Foreign Minister Sergey Lavrov on Friday. Blinken said he pushed Lavrov to accept the "substantial proposal" the U.S. put forth to free detained Americans Brittney Griner and Paul Whelan.

This marked the first time the two leaders spoke since the war began.

Blinken called it "a frank and direct conversation."

"I pressed the Kremlin to accept the substantial proposal that we put forth on the release of Paul Whelan and Brittney Griner,” he said. "I'm not going to characterize his responses and I can't give you an assessment of whether I think things are more or less likely."

On Russia's stated plans to annex parts of Ukraine, Blinken said he told Lavrov that "those plans will never be accepted. The world will not recognize annexations. We will pose additional significant costs on Russia if it moves forward with its plans."

Blinken did reaffirm that their call did not cover brokering peace in Ukraine.

"Unfortunately, tragically, we've seen no opening--willingness on the part of Russia to engage meaningfully on ending the aggression," Blinken lamented. "At the same time, I've also said that if there are issues where we could make a difference in senior Russians hearing directly from me or from colleagues, we would of course, pursue that."

The Russian Foreign Ministry confirmed the conversation and said it was initiated by the U.S.

The Russian Foreign Ministry said, "Regarding the possible exchange of imprisoned citizens of Russia and the United States, the Russian side is urged to return to the mode of professional, without speculative information stuffing, dialogue in the mode of 'quiet diplomacy.'"

White House spokesperson John Kirby said Friday, "We very much tried to keep details of the negotiations of this nature as private as possible so that we can allow as much space for negotiators to come to a successful conclusion. We felt that in the context of what was happening in both Brittney Griner and Paul Whelan's cases, as well as what was not happening, that it was important to lay out publicly that there was, in fact, a serious offer made by the American side that has not been acted on."

-ABC News’ Shannon Crawford and Molly Nagle

The U.S. doesn’t consider fighting in eastern Ukraine "stalled," White House spokesman John Kirby said Friday.

"I don't believe we consider what's going on in the Donbas is stalled," he said in response to a reporter’s question. “There's a lot of activity still going on in the Donbas."

He did note that over the last "48 to 72 hours, there has not been that many significant changes in the battle lines" in the east and the south.

"It can be true that in some places in eastern and southern Ukraine, the Russians appear to be taking a knee, refitting, refreshing, moving troops around,. And in other places in the east and in the south, there's actual battle going on between Russian and Ukrainian forces," Kirby said.

"There are units that are in actual contact with one another, and there are days where the Russians make a mile or two, and then there are days when the Ukrainians push them back a mile or two. And there's days when the Ukrainians are going on the counter-offensive on a tactical level and then they will stop to reconsider their next move," he said.

-ABC News’ Ben Gittleson

U.S. Ambassador to Ukraine Bridget Brink told ABC News on Friday morning that she is "optimistic" ships carrying grain will begin leaving Ukraine this weekend, but that it's up to Russia to keep its side of the deal.

During an interview in the Ukrainian port city of Odesa, Brink told ABC News there is currently "no obstacle" to the ships' departure.

"Ukraine is ready to ship this grain, but it's also up to Russia to agree on the grain corridor and the ships will be ready to go," Brink said. ""It can be done, it should be done and, in fact, it must be done."

She declined to say whether the United States would impose consequences on Russia if it disrupted the United Nations-brokered deal or attacked the ships. But she underlined her country's support for Ukraine and the deal, saying it was important that Ukrainian grain starts reaching countries that need it.

"Twenty-four hours after the deal was agreed a week ago today, Russia bombed this very port where we're standing," Brink noted. "So I think it's imperative on Russia to live up to its commitments and to implement the agreement it signed onto, and imperative on all of us to ensure that Russia lives up to those commitments."

When asked if there was a "Plan B" if the deal failed, Brink said the focus was on doing everything to ensure "Plan A" works.

Earlier Friday, Brink and ambassadors of other G-7 countries held a press conference in Odesa while overseeing the preparations. She told reporters that she hopes an agreement confirming the safe corridors of the grain ships to sail through this weekend would be reached. Under the deal, Ukraine and Russia have been negotiating the precise routes the vessels will take across the Black Sea.

Since Russian forces invaded neighboring Ukraine on Feb. 24, the cost of grain has skyrocketed worldwide. Russia and Ukraine -- often referred to collectively as Europe's breadbasket -- produce a third of the global supply of wheat and barley, but Kyiv has been unable to ship exports due to Moscow's offensive. Last month, the Ukrainian Grain Association warned that Ukraine's wheat harvest is expected to plummet by 40%.

In recent weeks, there has been an all-out push from the U.S. and the U.N. to facilitate exports from war-torn Ukraine, desperate to offset what they foretell is a looming global food crisis with the potential to devastate the developing world. A Russian blockade in the Black Sea, along with Ukrainian naval mines, have made exporting siloed grain and other foodstuffs virtually impossible and, as a result, millions of people around the world -- particularly in Africa and the Middle East -- are now on the brink of famine.

-ABC News' Patrick Reevell