Russian forces have moved to the outskirts of Ukraine’s capital as US officials warned that President Vladimir Putin may be intent on installing a new, more friendly government.

The invasion began on Thursday morning 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.

US President Joe Biden is due to meet fellow leaders of Nato governments in what the White House described as an “extraordinary virtual summit” to discuss Ukraine.

Ukrainian President Volodymyr Zelensky vowed that his military will keep fighting back and he ordered a full mobilisation. He said 137 people – 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 three 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.

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

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

The Ukrainian military on Friday morning reported significant fighting in the area of Ivankiv, about 40 miles north-west 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.

Mr Zelensky said he has information that he is the number one target for the invading Russians but said he planned to remain in Kyiv.

The Russian military later said it had seized of a strategic airport just outside Kyiv and cut the city off from the west.

The Ukrainian military said a group of Russian spies and saboteurs had been seen in a district on the outskirts of Kyiv, and police told people not to exit a subway station in the city centre because there was gunfire in the area.

– Ukraine’s offer of neutrality

President Zelensky made another push for a diplomatic end to the fighting by offering to discuss with Russia the possibility of designating Ukraine a non-aligned country.

The Kremlin said it would analyse the offer even as Russian troops continued their offensive on Kyiv. Western nations had rejected Moscow’s demand for a halt to Ukraine’s bid to join Nato over fears that the alliance would pose a risk to Russia’s security. It was that rejection that Mr Putin used to justify the invasion, claiming the West had left him no other choice.

Kremlin spokesman Dmitry Peskov described Mr Zelensky’s offer as “a move in a positive direction” which the Russian government had taken note of and now needed to “analyse”.

But foreign minister Sergei Lavrov said Mr Zelensky “is simply lying” about his offer because he “missed the opportunity” to discuss Ukraine’s neutral status when Mr Putin proposed it earlier.

– 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 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, Mr 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 US had predicted Mr Putin would falsely claim that the rebel-held regions were under attack to justify an invasion.

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

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

– What sanctions are Western powers imposing?

In announcing a new round of sanctions on Thursday, Mr Biden said the US 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 Mr Putin’s inner circle, while also imposing export controls that would aim to starve Russia’s industries and military of US semiconductors and other high-tech products.

New US 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.

Prime Minister Boris Johnson said Britain would aim to cut Russia off from the UK’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 UK 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 UK airports.

The European Union and other Western allies, including Australia, Japan and South Korea, 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. An extraordinary meeting of the Uefa executive committee will be held on Friday.

Formula One chiefs dropped this season’s Russian Grand Prix at Sochi. The F1 race was not due until September but the motorsport series leadership decided it would be “impossible” to stage the Grand Prix after talks with teams and the FIA governing body. American team Haas also dropped the sponsorship of Russian company Uralkali during preseason testing in Barcelona.

Valery Gergiev, a conductor who is close to Mr Putin, will not lead the Vienna Philharmonic in a five-concert US tour that starts at Carnegie Hall on Friday.

Milan’s Teatro alla Scala sent a letter to Mr Gergiev asking him to make a clear statement in favour 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.