In pictures: The human cost of modern warfare as Russian invasion sets Ukraine ablaze

Russian forces have advanced towards major Ukrainian cities, forcing people to flee in their thousands to neighbouring countries.

Big explosions have been heard in Kyiv, Kharkiv, and Odessa as world leaders condemned the invasion that could potentially lead to mass casualties and topple Ukraine’s democratically elected government, while threatening the post-Cold War balance.

Ukraine’s president Volodymyr Zelensky declared martial law after Russian troops began their offensive and called on citizens to take up arms against their invaders.

Mr Zelensky said Ukraine’s armed forces were successfully fighting back against Russian troops and had “derailed” Moscow’s plan to take Kyiv and install a puppet government.

“We have withstood and are successfully repelling enemy attacks. The fighting goes on,” Mr Zelensky said.

The Ukrainian preisdent declined an offer from the US to be evacuated as he wanted to support efforts in Kyiv, according to a US intelligence source.

Three children were among the 198 Ukrainians killed since Russia’s invasion began, the Ukraine government said.

Vladimir Putin announced what he called the invasion during a televised address on Thursday morning, saying the attack was intended to protect civilians in eastern Ukraine – a false claim the US had predicted would be made the pretext for an invasion.

Mr Putin accused the US and its allies of ignoring Russia‘s demands to prevent Ukraine from joining Nato and for security guarantees. He also claimed that Russia does not intend to occupy Ukraine but will move to “demilitarise” it and bring those who committed crimes to justice.

Russia’s military said it was targeting Ukrainian air bases and other military structures, not the populated areas, but troops have since made advances on several cities.

Ukraine responded with an attack on a Russian air base in Millerovo, according to Russian media.

US president Joe Biden denounced Russia’s action as an “unprovoked and unjustified attack”, declaring “the world will hold Russia accountable”.

The US has announced harsh sanctions targeting Russia’s banks and influential individuals. The UK has made personal sanctions against Mr Putin and his defnece secretary, Sergey Lavrov, among other measures.

The European Union agreed the “strongest, the harshest package” of sanctions on Friday.

A senior Russian official has warned that Moscow could react to sanctions by opting out of the last remaining nuclear arms pact and freezing Western assets.

Around 120,00 refugees have fled Ukraine to neighbouring Poland, Moldova, Hungary, Romania and Slovakia, the UN said. The largest numbers were arriving in Poland, where 2 million Ukrainians have already settled to work in recent years. At the Medyka border crossing, the line of vehicles waiting to enter Poland stretched 9 miles into Ukraine. Poland declared its border open to fleeing Ukrainians, as did Hungary. “We will help everyone,” the Polish Border Agency said. “We will not leave anyone without help.”