War in PICS: Ukraine says 20,000 escape besieged Mariupol; UK says Russian forces struggling

Moscow has not captured any of Ukraine's 10 biggest cities following its invasion that began on February 24.

About 20,000 people have managed to escape the besieged port of Mariupol in private cars, the Ukrainian interior ministry said on Wednesday, but hundreds of thousands remain trapped by Russian shelling, many without heating, power or running water.

Russian forces have focused on Mariupol, constantly bombarding it for the past two weeks, as it is a key city on the Azov Sea coast which they must gain control of to push further west.

Ukrainian officials estimate that more than 2,500 residents have been killed in the fighting and at least 200,000 are in urgent need of evacuation.

Moscow has not captured any of Ukraine's 10 biggest cities following its invasion that began on February 24, the largest assault on a European state since 1945.

Russian forces are struggling to overcome the challenges posed by Ukraine's terrain, according to Britain's Ministry of Defence in an intelligence report on Wednesday.

Russian troops have remained largely tied to Ukraine's road network and have demonstrated a reluctance to conduct off-road manoeuvres. The destruction of bridges by Ukrainian forces has also played a key role in stalling Russia's advance, it said.

And Russia's continued failure to gain control of the air has drastically limited its options.

"The tactics of the Ukrainian Armed Forces have adeptly exploited Russia's lack of manoeuvre, frustrating the Russian advance and inflicting heavy losses on the invading forces," said the report.

Russia calls its actions a "special military operation" to demilitarise and "denazify" Ukraine. Ukraine and Western allies call this a baseless pretext for a war that has raised fears of wider conflict in Europe.