Russian forces haven't yet been able to capture any Ukrainian cities during their invasion of the country, according to a senior US defense official.

According to the report, the US official who was not named by the outlet said that Russian forces had not planned to bring enough fuel.

They have not made the progress that they wanted to make in the north. They have been frustrated by the determination of the resistance and it has slowed them down.

An unnamed US official told Fox News that they continue to believe that the resistance is greater than the Russians expected.

According to the Associated Press, the British Defense Ministry said that the speed of the Russian advance had slowed due to logistical difficulties and strong Ukrainian resistance.

A senior US defense official told the AP that the US believed more than half of the Russian troops in the country had entered the country.

The ministry said that Russian forces are leaving forces to encircle and isolated major Ukrainian population centers.

After weeks of warnings by the US, the Russian assault began on Thursday. Putin denied that an attack was imminent.

According to Britain and US intelligence, Russian forces were 19 miles outside of the Ukrainian capital city of Kyiv on Saturday night.

Russia has claimed that it only targeted military targets as part of the Ukrainian military operation, however, attacks have been reported to include apartment buildings, schools and bridges. A missile struck an apartment building in southwestern Kyiv early Saturday morning.

More than 1,000 people have been wounded and more than 200 have been killed, according to the health minister.