Explosions were heard early Sunday morning in the Ukrainian cities of Kyiv and Kharkiv, local media reported.
The explosions came after several hours of quiet in the Ukrainian capital.
Earlier, Ukraine's President Volodymyr Zelensky spoke to British Prime Minister Boris Johnson by telephone and told him that the next 24 hours are crucial for Ukraine, said a Downing Street spokesperson quoted by Reuters.
Johnson told Zelensky that the UK and its allies would do everything possible to guarantee that defensive aid reached Ukraine, the spokesperson added.
Meanwhile, the general staff of Ukraine's armed forces described Sunday as "a difficult time" for the military, and added that troops "continue shelling in almost all directions".
In an English-language post on Facebook, the general staff said defense force members in the Vasylkiv military air base south west of Kyiv were resisting artillery strikes and Russian attacks.
On Sunday night, Russian military forces took control of the city of Berdiansk in southeastern Ukraine, the city’s mayor said.
In addition, satellite imagery taken on Sunday showed a three-mile column of Russian troops and tanks "moving towards Kyiv".
The images released by Maxar Technologies, a space technology company, showed a large deployment of Russian ground troops moving in the direction of the Ukrainian capital from approximately 40 miles (64 km) away.
The deployment consisted of hundreds of military vehicles and extended more than 3.25 miles (5 km), Maxar said, according to The Telegraph.
The convoy was situated northeast of the Ukrainian city of Ivankiv and contained fuel, logistics and armored vehicles including tanks, infantry fighting vehicles and self-propelled artillery.
Earlier, Russian forces surrounded Kyiv, cutting off the country’s largest city.
The mayor of Kyiv, Vitali Klitschko, told The Associated Press that civilians could no longer be evacuated from the city, as it has been encircled by Russian soldiers.
“We can’t do that, because all ways are blocked,” he said. “Right now we are encircled.”
Klitschko said that since the Russian invasion last Thursday, nine civilians have been killed in the city of 2.8 million, including one child.
Negotiations between Ukraine and Russia are set to commence on Monday morning at Ukraine’s border with Belarus.
Zelensky admitted that he was not confident that any progress would be made, but he insisted that he would nonetheless make the attempt.
“I’ll say frankly, like always, that I don’t believe in the result of this meeting,” he said. “But let’s try, so that no citizen of Ukraine will have any doubt that I, as President, tried to stop the war when there was even a [slight] chance.”