Russia has broken Sunday’s temporary cease-fire in Mariupol, two officials with the mayor’s office told ABC News.

The officials said Russia continued shelling during the first two hours of the cease-fire, which had been scheduled to begin at 10 a.m. local time.

Mariupol officials earlier said an evacuation of residents would begin at noon local time. Sunday’s planned evacuation was to come after a failed cease-fire on Saturday, Ukrainian officials said.

-ABC News’ Matt Gutman

Secretary of State Antony Blinken on Sunday reacted to WNBA star Brittney Griner being detained in Russia, saying “whenever an American is detained anywhere in the world, we of course stand ready to provide every possible assistance and that includes in Russia.”

Blinken noted that he was limited in what he could say, given privacy considerations. He made the comments on Sunday alongside Moldovan President Maia Sandu at a press conference.

He also brought up efforts to seek the release of Paul Whelan and Trevor Reed, two Americas who’ve been detained by Russia.

-ABC News’ Justin Ryan Gomez

More than 1.5 million people have fled Ukraine since the Russian invasion began, the head of the U.N. Refugee Agency said on Sunday.

"More than 1.5 million refugees from Ukraine have crossed into neighbouring countries in 10 days -- the fastest growing refugee crisis in Europe since World War II," Filippo Grandi, the U.N. high commissioner for refugees, said on Twitter.

An official in Mariupol, a Ukrainian city besieged by the Russians, said a cease-fire on Sunday will allow citizens to evacuate.

The temporary cease-fire was scheduled to begin at 10 a.m. local time, with an evacuation starting two hours later, Pavlo Kyrylenko, governor of Donetsk Oblast, said on Facebook.

A similar plan had been announced for Saturday, but Russian shells continued to fall throughout the day.

The Russian forces charging toward Kyiv haven't made progress in the last day as they face Ukrainian resistance and low food and gas supply, a senior U.S. defense official told reporters Tuesday.

However, it could be a deliberate pause, the official said. "Part of the stall could be ... as a result of their own self-determined sort of pause in operations -- that they are possibly regrouping, rethinking, reevaluating," the official said.

The U.S. believes Russian forces "have committed now more than 80% of what was their pre-staged combat power," the official added.

The official said some Russian soldiers weren't told they were going into combat. The official said "not all of them were apparently fully trained and prepared."

The strong Ukrainian resistance has also hurt morale, according to the official.

Russia has now launched more than 400 missiles on Ukraine, the official said. The U.S. believes Russia has launchers that could be used for thermobaric weapons, but cannot confirm their use, the official said.

Russian forces are making the most progress in the south. Russians are attacking Kherson in south Ukraine, which "appears very much to be contested city at this point," the official said.

Russians are also approaching Mariupol in southeastern Ukraine, and while they haven't yet entered the city, "they are close enough now that they could attack Mariupol with long range fires," the official said.

Two towns on the path to Mariupol are believed to be occupied by the Russians, according to the official.

The U.S. believes the Russians hope to move north out of Mariupol up to the heavily-contested city of Kharkiv. The official said they believe Russian forces are trying to encircle Kharkiv.

The U.S. official noted that they've seen "certain risk-averse behavior by the Russian military" over the last week.

"Take the amphibious assault, for instance. They put those troops ashore a good 70 kilometers away from Mariupol because they knew Mariupol was going to be defended and they could put them ashore in an uncontested environment. And they still haven't reached Mariupol," the official said.

"They are not necessarily willing to take high risks with their own aircraft and their own pilots," the official said.

"And of course we're seeing that on the ground -- the fairly slow and steady progress that they have made, and you guys are seeing it for yourselves on the ground where ... units are surrendering, sometimes without a fight."

-ABC News' Matt Seyler