WARSAW, Poland (AP) — Russian forces moved to the outskirts of Ukraine's capital on Friday as U.S. officials warned that President Vladimir Putin may be intent on installing a new, more friendly government.

The invasion began early Thursday with a series of missile strikes, many on key government and military installations. They were quickly followed by a ground assault from the north, east and south in an attack that could rewrite the global post-Cold War security order.

U.S. President Joe Biden was to meet Friday morning with fellow leaders of NATO governments in what the White House described as an “extraordinary virtual summit” to discuss Ukraine.

Ukrainian President Volodymyr Zelenskyy vowed that his military will keep fighting back and he ordered a full mobilization. He said 137 people, both servicemen and civilians, have been killed and hundreds more wounded since the invasion began.

Here are the things to know about the conflict over Ukraine and the security crisis in Eastern Europe:

AN UNEASY NIGHT IN KYIV

Fearing a Russian attack, many of the capital’s residents took shelter deep underground in metro stations. People brought sleeping bags and blankets, dogs and crossword puzzles as they sought safety in the makeshift bomb shelters.

In the early hours of the morning, several explosions were heard in different parts of the city. Air raid sirens also went off.

Kyiv Mayor Vitaly Klitschko had called on the city’s 3 million people to stay indoors unless they worked in critical sectors and said everyone should prepare go-bags with necessities such as medicine and documents.

Friday morning, Klitschko said at least three people were injured when a rocket hit a multi-story apartment building, starting a fire.

“Just as yesterday, the military and civilians are equally under Russian attack,” Zelenskyy said.

The Ukrainian military on Friday morning reported significant fighting in the area of Ivankiv, about 60 kilometers (40 miles) northwest of Kyiv, as Russian forces apparently tried to advance on the capital from the north. It said one bridge across a small river had been destroyed.

Zelenskyy said he has information that he’s the No. 1 target for the invading Russians but said he planned to remain in Kyiv.

CHERNOBYL IN RUSSIAN HANDS

Ukraine said it lost control of the Chernobyl nuclear site after Ukrainian forces waged a fierce battle with Russian troops. A nuclear reactor at the plant 130 kilometers (80 miles) north of Kyiv exploded in April 1986, sending a radioactive cloud across Europe. The damaged reactor was later covered by a protective shell.

Alyona Shevtsova, an adviser to the commander of Ukraine’s Ground Forces, wrote on Facebook that the staff had been “taken hostage” when Russian troops seized the facility. The White House press secretary expressed alarm, concerned that it could hamper efforts to maintain the nuclear facility.

HOW HAS PUTIN JUSTIFIED THE INVASION?

In a televised address as the attack began, Putin said it was needed to protect civilians in eastern Ukraine, where Ukrainian forces and Russia-backed separatists have been fighting for almost eight years.

The U.S. had predicted Putin would falsely claim that the rebel-held regions were under attack to justify an invasion.

He accused the U.S. and its allies of ignoring Russia’s demands to prevent Ukraine from ever joining NATO and offer Moscow security guarantees.

Putin said Russia does not intend to occupy Ukraine but plans to “demilitarize” it, a euphemism for destroying its armed forces.

WHAT SANCTIONS ARE WESTERN POWERS IMPOSING?

In announcing a new round of sanctions on Thursday, Biden said the U.S. and its allies will block the assets of four large Russian banks, impose export controls and sanction oligarchs.

The penalties fall in line with the White House’s insistence that it would look to hit Russia’s financial system and Putin’s inner circle, while also imposing export controls that would aim to starve Russia’s industries and military of U.S. semiconductors and other high-tech products.

New U.S. sanctions also targeted the military and financial institutions of Belarus, which Russia is using as a staging ground for its troops moving into Ukraine from the north.

British Prime Minister Boris Johnson said he would aim to cut Russia off from the U.K.’s financial market.

The sanctions include freezing the assets of all major Russian banks, including VTB Bank, its second-biggest. Britain also plans to bar Russian companies and the Russian government from raising money on U.K. markets, ban the export of a wide range of high-tech products, including semiconductors, to Russia and bar its flagship airline, Aeroflot, from landing at U.K. airports.

The European Union and other Western allies, including Australia, Japan and South Koreas, announced similar sanctions.

OTHER REPERCUSSIONS FOR RUSSIANS

UEFA will no longer host the Champions League final in St. Petersburg in May, a person with knowledge of the decision told The Associated Press. The person spoke on condition of anonymity to discuss private talks. An extraordinary meeting of the UEFA executive committee will be held Friday.

Valery Gergiev, a conductor who is close to Putin, will not lead the Vienna Philharmonic in a five-concert U.S. tour that starts at Carnegie Hall on Friday. Milan’s Teatro alla Scala sent a letter to Gergiev asking him to make a clear statement in favor of a peaceful resolution in Ukraine or he would not be permitted to return for his next scheduled performance on March 5.

PROTESTS IN RUSSIA

Russians shocked by the invasion turned out by the thousands for street protests in Moscow and other cities. They signed open letters and online petitions demanding the Kremlin halt the assault. One petition garnered 330,000 signatures by the end of the day.

The crackdown was swift. Some 1,745 people in 54 Russian cities were detained, at least 957 of them in Moscow.

State television was all in for the invasion, with one host calling it an effort to protect people in eastern Ukraine from a “Nazi regime.”

CHINA’S SUPPORT FOR RUSSIA

China’s customs agency on Thursday approved imports of wheat from all regions of Russia, a move that could help to reduce the impact of possible Western sanctions.

China’s market is a growth area for other suppliers, but Beijing had barred imports until now from Russia’s main wheat-growing areas due to concern about possible fungus and other contamination.

Russia is one of the biggest wheat producers, but its exports would be vulnerable if its foreign markets blocked shipments in response to its attack on Ukraine.

Thursday’s announcement said Russia would “take all measures” to prevent contamination by wheat smut fungus and would suspend exports to China if it was found.

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Associated Press writers around the world contributed to this report.

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