KYIV, Ukraine

Russian soldiers have seized several Ukrainian army vehicles and are heading towards the center of the capital Kyiv, a top Ukrainian defense official said on Friday.

On social media, Deputy Defense Minister Anna Malyar said Russian soldiers had seized two Ukrainian vehicles as well as Ukrainian soldiers' uniforms and are advancing towards the Kyiv city center from Obolon, a northern residential district of the capital some 12 kilometers (7.5 miles) from the center.

"A convoy of Russian military trucks are following them. They will surely be neutralized," Malyar added.

Separately, according to a Ukrainian General Staff statement, the Ukrainian Air Force is protecting the skies of Kyiv, and since 4 a.m. (0200GMT) Ukraine's air defense system has blocked rocket attacks on the capital.

Sirens and several explosions were heard early Friday in both Kyiv and Lviv, a city in western Ukraine some 540 km (336 mi) from the capital.

‘Worst since World War II attacks by Nazis’

Ukrainian Foreign Minister Dmytro Kuleba said there were "horrific Russian rocket strikes on Kyiv."

"The last time our capital experienced anything like this was in 1941, when it was attacked by Nazi Germany," he said on Twitter early Friday.

"Ukraine defeated that evil and will defeat this one. Stop Putin. Isolate Russia. Sever all ties. Kick Russia out of everywhere," he added.

There were also local reports of a warplane being downed over Kyiv, initially thought to be a Russian aircraft but later was announced to be Ukrainian.

In addition to airstrikes, Russia is also pressuring the city on the ground, with the Ukrainian army blowing up three bridges around Kyiv to prevent the advance of Russian tanks.

Anton Gerashchenko, adviser to the Ukrainian interior minister, said a tank convoy of Russian military units was about to enter the capital via Ivankiv and Chernihiv, two cities to the northwest and north of the capital.

"Today is going to be a very difficult day," Gerashchenko said, adding that Russian tanks could be destroyed with Ukrainian anti-tank weapons.

Later in the day, Yuri Galushkin, the Territorial Defense Course commander, said on Twitter that procedures for joining the nation’s military had been simplified.

“Just bring your passport and ID number. There are no age restrictions,” he added.

Meanwhile, US officials said that Russian troops are currently only 30-35 km (20-21 mi) away from Kyiv.

Following a months-long buildup of troops and equipment around neighboring Ukraine’s borders, Russian President Vladimir Putin on Thursday announced a military intervention into Ukraine.

The attack has been met by international condemnation and announcements of new sanctions by Western countries.