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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.

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.