The mayor of the Ukrainian capital, Kyiv, has ordered citizens to stay indoors until Monday morning as Russian forces close in on the city.
A curfew imposed at 15:00 GMT on Saturday is to stay in force until 06:00 GMT on Monday.
All those venturing out "will be considered members of sabotage groups of the enemy", authorities warned.
The UK's Ministry of Defence (MoD) also said that Russian saboteurs were thought to be in Kyiv.
The latest developments came on the third day of the Russian assault on Ukraine, as Russian forces continued to bombard the capital and other cities with artillery and missiles.
However, Western intelligence sources pointed to evidence that the invaders were not making as much progress as they had expected.
"The speed of the Russian advance has temporarily slowed, likely as a result of acute logistical difficulties and strong Ukrainian resistance," the MoD said in a tweet.
"Russian forces are bypassing major Ukrainian population centres while leaving forces to encircle and isolate them.
"Overnight clashes in Kyiv are likely to have involved limited numbers of pre-positioned Russian sabotage groups."
The capture of Kyiv remained Russia's main objective, the ministry added.
According to US sources, Russia has so far fired more than 250 missiles, mostly short-range, at Ukrainian targets, some of which have hit civilian infrastructure.
Early on Saturday, an apartment building in Kyiv was struck by a missile, leaving a hole covering at least five floors. It is thought that most of the occupants were in shelters.
President Volodymyr Zelensky said Ukraine had been fighting Russian troops in Kyiv, as well as Odesa in the south and Kharkiv in the north-east, but the capital was still in Ukrainian hands.
"The occupiers wanted to block the centre of our state," the president said. "We broke their plan."
In other developments:
- Ukraine says it has repulsed an attack on the northern city of Kharkiv after heavy fighting, but shelling of Okhtyrka nearby has caused many casualties.
- The invasion has so far killed 198 Ukrainians, the health minister says.
- Russia says it has captured the city of Melitopol in the south.
- The UN estimates that 120,000 people have fled Ukraine in the past 48 hours.
- French police have impounded a Russian-flagged cargo ship suspected of breaching sanctions imposed because of the Ukraine war.
- Germany has approved delivery of 1,000 anti-tank rocket weapons to Ukraine, in a change to its long-standing policy of banning weapon exports to conflict zones.
Western analysts estimate that Russia's initial assault involved about half of the more than 150,000 troops it had massed on the border in the run-up to the invasion.
BBC defence correspondent Jonathan Beale says Russia has also not used its artillery and air strikes as intensively as expected.
However, our correspondent adds that it is normal for militaries to keep reserves as they adjust plans. Russia may need them for later phases of the invasion, he says.