A man walks through the site of a blast targeted at the TV tower, as Russia's invasion of Ukraine continues, in Kyiv
Kherson, a shipbuilding center and a major Ukrainian city, has fallen into Russian command. In photo: a man walks through the site of a blast targeted at the TV tower, as Russia's invasion of Ukraine continues, in Kyiv, Ukraine March 2, 2022. Reuters / UMIT BEKTAS

Russian forces appear to have become more aggressive in their targeting of infrastructure inside Kyiv, which has seen an increase of missile and artillery strikes, as well as throughout the country, a senior U.S. official said on Wednesday.

After nearly a week, Russia has yet to achieve its aim of overthrowing Ukraine's government and its claims to have taken a first city were disputed by Ukraine and Washington.

But the war's toll is increasingly dealing a heavy blow to civilians, with Ukrainian authorities estimating more than 2,000 civilians killed so far as Russian strikes destroy hospitals, schools and homes.

"We've observed, certainly as you have all observed, an increase in missiles and artillery that (is) targeting the city and this (increasing) aggressiveness in terms of just the iron that they're lobbing into the city," the official said.

After failing to swiftly take major cities and to subdue Ukraine's military, U.S. officials have said for days that they believe that Russia will move toward shifting its strategy to encircling cities, cutting off supply and escape routes, then attacking with a combined force of armor, ground troops and engineers.

The most intensive bombardment has struck Kharkiv, a city of 1.5 million people in the east, whose center has been turned into a bombed-out wasteland of ruined buildings and debris.

The official stopped short of directly accusing Russian troops of specifically targeting civilians.

"The worry is that as they become more aggressive, they will become less precise and less discriminant," the official said.

British Prime Minister Boris Johnson said on Wednesday that he believed the actions of Russian President Vladimir Putin toward Ukraine already qualified as a war crime.

The official said that 450 missiles had been launched against Ukrainian targets over the past seven days by Russian forces.

Moscow said on Wednesday it had captured Kherson, a provincial capital of around a quarter of a million people on the southern front, but Ukraine disputed the claim.

The official said that despite the Russian claims, the city was still being fought for by the Ukrainians.