Russia's invasion of Ukraine entered its 12th day following what Ukrainian authorities described as increased shelling of encircled cities and another failed attempt to evacuate civilians from the besieged southern port of Mariupol.

Russian and Ukrainian forces had agreed to an 11-hour cease-fire Sunday, but Ukrainian officials said Russian attacks quickly closed the safe-passage corridor.

A third round of talks between Russian and Ukrainian leaders was planned for Monday.

More than 1.5 million Ukrainians had been forced from the country. Ukrainian President Volodymyr Zelenskyy urged his people to keep resisting, and Ukraine's foreign minister said more than 20,000 people from 52 countries had volunteered to fight in Ukraine's newly created international legion.

Russian President Vladimir Putin likened the West's sanctions on Russia to "declaring war."

Here's a look at key things to know about the conflict:

Violence Stops Planned Civilian Evacuations Again

Ukrainian Interior Ministry adviser Anton Gerashchenko blamed Russian artillery fire for halting a second attempt in as many days to evacuate civilians from Mariupol, where food, water and medicine are scarce.

A day earlier, Ukrainian officials similarly said Russian artillery fire and airstrikes had prevented residents from leaving. Putin accused Ukraine of sabotaging the effort.

Russia has sought to cut off Ukraine's access to the Sea of Azov in the south. Capturing Mariupol could allow Russia to establish a land corridor to Crimea, which it annexed in 2014.

What else is happening on the ground?

Russian forces launched hundreds of missiles and artillery attacks across the country, including powerful bombs dropped on residential areas of Chernihiv, a city north of the capital of Kyiv, Ukrainian officials said. But a miles-long Russian armored column threatening the capital remained stalled outside Kyiv.

Sunday evening, heavy shelling also came to Mykolaiv in the south and Kharkiv, the country's second-largest city. Efforts to evacuate residents from the Kyiv suburbs of Bucha, Hostomel and Irpin on Sunday were mostly unsuccessful.

A senior American defense official said Sunday the U.S. believes that about 95% of the Russian forces that had been arrayed around Ukraine are now inside the country. Ukrainian air and missile defenses remain effective and in use, and the Ukrainian military continues to fly aircraft and to employ air defense assets, the official said.

Ukrainian forces were also defending Odesa, Ukraine's largest port city, from Russian ships, Ukrainian presidential adviser Oleksiy Arestovich said.

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The Russian Defense Ministry on Sunday announced plans to strike Ukraine's military-industrial complex, and it alleged that Ukrainian forces were plotting to blow up an experimental nuclear reactor in Kharkiv and to blame it on Russia. The ministry offered no evidence to back its claims, which could not be independently verified.

Zelenskyy Pushes Call For No-Fly Zone

Zelenskyy pushed his call for foreign countries to impose a no-fly zone over Ukraine. Establishing a no-fly zone would risk escalating the conflict by involving foreign militaries directly. Although the United States and many Western countries have backed Ukraine with weapons shipments, they have sent no troops.

Zelenskyy said in a video address on Sunday that "the world is strong enough to close our skies" and over the weekend he urged U.S. officials help his country obtain warplanes to fight the invasion and retain control of its airspace.

Russian Defense Ministry spokesman Igor Konashenkov said Sunday that some Ukrainian combat planes had redeployed to Romania and other Ukraine neighbors he didn't identify. He warned an attack from planes operating out of those nations could be deemed an engagement by them in the conflict.

Directly Witnessed Or Confirmed By The Associated Press

Onlookers in Chernihiv cheered as a Russian military plane fell from the sky and crashed, according to video released by the Ukrainian government. In Kherson, hundreds of protesters waved blue and yellow Ukrainian flags and shouted, "Go home."

In Mariupol, Associated Press journalists saw doctors make futile attempts to save wounded children. Pharmacies ran bare and hundreds of thousands of people faced food and water shortages in freezing weather.

In Irpin, near Kyiv, a sea of people on foot and even in wheelbarrows trudged over the remains of a destroyed bridge to cross a river and leave the city. Assisted by Ukrainian soldiers, they lugged pets, infants, purses and flimsy bags stuffed with minimal possessions. Some of the weak and elderly were carried along the path in blankets and carts.

Kyiv's central train station remained crowded with people desperate to leave, and frequent shelling could be heard from the center of the capital city.

Diplomatic Efforts

Intense diplomatic efforts continued, with U.S. Secretary of State Antony Blinken in Moldova pledging America's support to the small Western-leaning former Soviet republic. The country is coping with an influx of refugees from Ukraine and keeping an eye on Russia's intensifying war with its neighbor.

Blinken says the United States and its allies are having a "very active discussion" about banning the import of Russian oil and natural gas.

In a call with Putin that lasted nearly two hours on Sunday, French President Emmanuel Macron repeated calls for Russia to halt military operations, protect civilians and allow humanitarian aid. A French official reported that Putin said he does not intend to attack nuclear plants.

The director general of the International Atomic Energy Agency, Rafael Mariano Grossi, said Sunday that Ukrainian staff at the country's largest nuclear plant are now required to seek approval for any operation, even maintenance, from the Russians. The Zaporizhzhya plant was seized by the Russians last week.

Putin continued to blame the war on the Ukrainian leadership, saying, "They are calling into question the future of Ukrainian statehood." In a call with Turkish President Recep Tayyip Erdogan on Sunday, Putin said the invasion could be halted only "only if Kyiv ceases hostilities," according to a Kremlin account.

Israel's prime minister spoke with Putin on Sunday, a day after they met directly in Russia. Israel is one of the few countries that has good working relations with both Russia and Ukraine.

The Humanitarian Situation

The death toll of the conflict has been difficult to measure. The U.N. human rights office said at least 364 civilians have been confirmed killed since the Feb. 24 invasion, but the true number is probably much higher.

The World Health Organization said it verified at least six attacks that have killed six health care workers and injured 11 others.

The U.N. World Food Program says millions of people inside Ukraine, a major global wheat supplier, need food aid "immediately."

Ukrainian refugees continued to pour into neighboring countries, including Poland, Romania and Moldova. The number of people who have left since fighting began has now reached 1.5 million, according to U.N. refugee agency.

BUSINESS IN RUSSIA

A growing number of multinational businesses have cut off Russia from vital financial services, technology and a variety of consumer products in response to Western economic sanctions and global outrage over the war.

Two of the so-called Big Four accounting firms - KPMG and PricewaterhouseCoopers - said Sunday they were pulling out of Russia, ending relationships with member firms based in the country.

TikTok said users won't be able to post new videos in Russia in response to the government's crackdown on what people can say on social media about the invasion, and American Express announced it was suspending all operations in Russia and Belarus.

Netflix also announced it was suspending its service in Russia.