GENEVA (REUTERS) - More than 100 buses carrying several thousand civilians have left the besieged city of Sumy in northeastern Ukraine, in a "safe passage" operation, the International Committee of the Red Cross (ICRC) said on Tuesday (March 15).

The buses bearing the Red Cross emblem were heading towards Poltava, in central Ukraine, but might not be able to take a direct route, ICRC spokesman Jason Straziuso said, adding the Russian side had given a green light for the evacuation.

"In total more than 100 buses are travelling in two different convoys, a joint operation between us and the Ukrainian Red Cross," Straziuso told Reuters in Geneva.

ICRC spokesman Ewan Watson told a UN briefing earlier that humanitarian corridors had not always been respected.

"That is really up to the parties to the conflict to agree on the terms of a safe passage agreement and then stick to it," Watson said, adding that there had been delays with a similar evacuation from the southeastern city of Mariupol.

"But the bottom line here is that hundreds of thousands of people remain without aid today. They are unable to leave the city (Mariupol) today and they are essentially being suffocated in this city right now with no aid and with a dire situation and very much dwindling supplies," Watson said.

Some three million Ukrainians have fled their homeland since Russian forces invaded 20 days ago, Paul Dillon of the UN International Organisation for Migration (IOM) said.

They include 1.8 million Ukrainians now in Poland, to UN refugee agency said, with 300,000 having moved to Western Europe.

Among the refugees are 1.4 million children, meaning 73,000 children have become refugees on average each day since the invasion - almost one per second, Unicef spokesman James Elder said.

"Ukrainian children arriving in neighbouring countries are at significant risk of family separation, violence, sexual exploitation, and trafficking," Elder said.