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.

While Poland has been welcoming refugees fleeing the destruction in Ukraine, the country will eventually meet its limit on how many people it can take in, Konrad Fijolek, president of Rzeszow, Poland, a city about 60 miles from the Ukrainian border, told ABC News.

Like many cities that border Ukraine, Rzeszow, a town of about 180,000 people, has become a pathway to safety and a lifeline for millions of refugees flooding across the border from Ukraine.

Poland will be able to receive about 1.5 million people, but any more will put the country under strain, Fijolek said.

"If a million or more come, we won't be able to cope," he said.

Some estimates suggest as many as 4 million people could leave Ukraine due to the conflict, but most of the major cities in the country are already full — and officials are attempting to move refugees into smaller cities, a feat that is "not easy," Fijolek said.

So far, all the shelters set up in Poland are temporary, he added.

"We would like to avoid the view of refugee camps here," he said. "We would like people who are escaping from the war to think that what awaits here is proper camp not camps."

In addition, the mental state among the people in Poland is continuing to deteriorate, Fijolek said.

"We as local leaders would like to send the message that if there's any possible action to stop the war [find it] and then try to negotiate, because every day at the border we can see the human tragedy," he said.

-ABC News' Chris Donato and Marcus Moore