• "The enemy has been stopped in most directions. The fights continue," Zelenskyy said in a speech that was translated by NBC News.
• Ukraine's army has about 145,000-150,000 troops, according to a January report from the U.S. Congressional Research Service.
• The situation on the ground in Ukraine is fluid, and accounts of the military situation are difficult or impossible to confirm.

Ukraine's military has stopped Russian invasion troops "in most directions" despite renewed missile strikes that began at 4 a.m. local time, Ukrainian President Volodymyr Zelenskyy said Friday.

"The enemy has been stopped in most directions. The fights continue," Zelenskyy said in a speech that was translated by NBC News. "Russia expects us to get tired, but we're not tired."

The situation on the ground in Ukraine is extremely fluid, and accounts of the military situation are difficult or impossible to confirm.

Russia is believed to have 190,000 troops in the vicinity of Ukraine, but it's not known how many of them have invaded Ukrainian territory.

Ukraine's army has about 145,000-150,000 troops, according to a January report from the U.S. Congressional Research Service.

Many of Ukraine's rank-and-file infantry have gained combat experience against pro-Russian separatists in the eastern part of the country, where low-level fighting has been ongoing since 2014.

Separately, the country is believed to be in the process of mobilizing thousands of reservists.

U.K. Defense Secretary Ben Wallace told Sky News on Friday that Russia had lost more than 450 personnel, and Russian special forces had failed in an attempt to take a key airport from Ukrainian defenders.

"Our assessment, as of this morning, is that Russia has not taken any of its major objectives," Wallace said. "In fact, it is behind its hoped-for timetable."

Air defenses

Zelenskyy said Ukraine's air defenses are "working and protecting our skies," adding that Russian aircraft had been seen in Kyiv on Friday morning.

Kyiv was subjected to rocket strikes on Friday, according to its foreign minister. A live Reuters video stream of Kyiv showed the city's streets mostly empty, punctuated by air raid sirens.

The State Emergency Service of Ukraine provided footage of firefighters at the scene of a heavily damaged building that was struck by what it said was a downed Russian aircraft.

"There were several explosions this morning," Zelenskyy said Friday. "This is the same situation as happened to Ukraine in 1941. This morning we protect our state on our own, as the world's countries look to us."

Ukraine was the scene of vicious fighting against invading troops from Nazi Germany in the 1940s.

— CNBC's Victor Loh contributed to this report.