Russian President Vladimir Putin put his nuclear forces on increased alert on Sunday in an unprecedented escalation of tensions with the West. Russia’s massive conventional military assault on Ukraine entered its fourth day with fighting in the streets of the country’s second-largest city.

Here are the things to know about the Russian invasion of Ukraine and the security crisis in Eastern Europe:

RUSSIA PUTS NUCLEAR FORCES ON ALERT

In a shocking move that immediately unearthed fears many thought permanently buried from the Cold War, Putin ordered Russian nuclear weapons prepared for increased readiness to launch, ratcheting up tensions with Europe and the United States over the conflict that is dangerously poised to expand beyond the former frontiers of the defunct Soviet Union.

The Russian president told his defense minister and the chief of the military’s General Staff to put the nuclear deterrent forces in “special regime of combat duty.”

He said that leading NATO powers had made “aggressive statements” toward Russia in addition to stiff economic sanctions and cutting leading Russian banks from the SWIFT banking system.

RUSSIA AND UKRAINE TO HOLD TALKS

After rejecting Putin’s offer to meet in the Belarusian city of Homel on the grounds that Belarus was facilitating the Russian assault, Ukrainian President Volodymyr Zelenskyy agreed to send a Ukrainian delegation to meet with Russian counterparts at an unspecified time and location on the Belarusian border.

The announcement comes hours after Russia announced that its delegation had flown to Belarus to await talks. Ukrainian officials initially rejected the move, saying any talks should take place elsewhere than Belarus, a country that has allowed Russia to use its territory as a staging ground for the invasion of Ukraine.

Zelenskyy, who has refused to abandon the Ukranian capital of Kyiv, named Warsaw, Bratislava, Istanbul, Budapest or Baku as alternative venues for talks, before accepting the Belarus border.

The Kremlin added later that Israeli Prime Minister Naftali Bennett had offered to help broker an end to fighting in a call with Putin. It didn’t say whether the Russian leader accepted.

FIGHTING SPREADS IN UKRAINE

Attempting to lay siege to Ukraine’s capital of Kyiv, Russian forces focused on cutting off the country’s southern coast and isolating it from the sea, while also probing the inner defenses of Kharkiv, a Ukrainian city close to the Russian border.

Ukrainians awoke Sunday to street fighting in the northeastern city of 1.4 million located only 20 kilometers (12.4 miles) south of the border with Russia. Regional authorities administration said that Ukrainian forces had engaged Russian troops inside the city and asked civilians not to leave their homes.

The immediate fate of the Russian advance was uncertain, but with Ukrainians volunteering en masse to fight back alongside regular army units, it seemed that the city’s defenses offered stiff resistance. Having already ordered men between 18-60 years old not to leave the country, Ukrainian officials said Sunday that they were releasing prisoners with military experience who want to take up arms for their country.

The ground attack came after Russians blew up a gas pipeline in Kharkiv, according to Ukrainian officials who ordered residents to cover their windows with a damp cloth given the “environmental catastrophe” it posed.

Britain’s defense ministry said while overnight skirmishing in Kyiv had been less intense than on Friday night, Russian forces were attempting to encircle the city. The capital remains under lockdown after its mayor called a curfew from 5 p.m. Saturday until 8 a.m. Monday.

Kyiv Mayor Vitali Klitschko on Sunday told The Associated Press that some of the Russian attackers are within 20 kilometers (12 miles) of the city center and that he feared about a long siege.

“Right now, we have electricity, right now we have water, and heating in our houses. But the infrastructure is destroyed … I guess we have not so much time,” Klitschko said.

MANY UKRAINIANS FLEE, SOME RETURN TO FIGHT

Those fleeing Europe’s largest armed conflict since World War II grew to 368,000 Ukrainians — mostly women and children — who have reached neighboring countries, the United Nations’ refugee agency said. That figure more than doubles the agency’s estimate from the day before.

The line of vehicles at the Poland-Ukraine border stretched 14 kilometers (8.7 miles), and those fleeing had to endure long waits in freezing temperatures overnight. Over 100,000 people have crossed into Poland alone, according to Polish officials.

Amid the rush to escape the bombs and tanks, there was a trickle of brave men and women who want to head home to defend Ukraine or help others do so.

At a border crossing in southern Poland, Associated Press journalists spoke to people in a line heading against the tide. They included a group of some 20 Ukrainian truck drivers who worked in Europe and wanted to face combat.

GERMANY ANNOUNCES SIGNIFICANT MILITARY SHIFT

A day after Germany announced it would send military aid to Ukraine, German Chancellor Olaf Scholz said that his government will increase its own defense spending to rearm amid the uncertainty of the extent of Putin’s ambitions.

Scholz’s pledge to dedicate 100 billion euros ($113 billion) to a special fund for its armed forces would raise Germany’s defense spending above 2% of GDP, finally satisfying a longstanding request by NATO allies for Europe’s largest economy to do more for the continent’s security.

Germany announced Saturday that it would send 1,000 anti-tank weapons and 500 Stinger surface-to-air missiles to Ukraine. Those weapons are in addition to the 400 German-made anti-tank weapons Germany also approved to be shipped from the Netherlands, as well as 9 D-30 howitzers and ammunition from Estonia.

The U.S. has also pledged an additional $350 million in military assistance to Ukraine, including anti-tank weapons, body armor and small arms.

RUSSIANS AGAINST THE WAR

Defying crackdowns by police, demonstrators marched in city centers from from Moscow to Siberia chanting “No to war!”

In St. Petersburg, where several hundred gathered in the city center, police in full riot gear were grabbing one protester after another and dragging some into police vans, even though the demonstration was peaceful. Footage from Moscow showed police throwing several female protesters on the ground before dragging them away.

According to the OVD-Info rights group that tracks political arrests, by Sunday evening police detained at least 1,474 Russians in 45 cities over anti-war demonstrations that day.

“I have two sons and I don’t want to give them to that bloody monster. War is a tragedy for all of us,” 48-year-old Dmitry Maltsev, who joined the rally in St. Petersburg, told The Associated Press.

MORE MOVES TO PUNISH RUSSIA

The European Union’s chief executive says the 27-nation bloc will close its airspace to Russian airlines, fund supplies of weapons to Ukraine and ban some pro-Kremlin media outlets in response to Russia’s invasion.

European Commission President Ursula von der Leyen said Sunday that “for the first time ever, the European Union will finance the purchase and delivery of weapons and other equipment to a country that is under attack.”

Von der Leyen added that “we are shutting down the EU airspace for Russians. We are proposing a prohibition on all Russian-owned, Russian registered or Russian-controlled aircraft. These aircraft will no more be able to land in, take off or overfly the territory of the EU.”

She said also the EU will ban “the Kremlin’s media machine. The state-owned Russia Today and Sputnik, as well as their subsidiaries, will no longer be able to spread their lies to justify Putin’s war and to sow division in our union.”

Japan joined the U.S. and European nations in cutting top Russian banks off from the SWIFT international financial messaging system. Japan will also send $100 million in emergency humanitarian aid to Ukraine.

(AP)