In the conflict in Ukraine, the world's attention is shifting to Russia's response to the refusal of Ukrainian forces in the besieged city of Mariupol to surrender.

Russia claimed that it had surrounded the steelworks that Ukrainian forces have been using as a base.

Russia also demanded that Ukrainians lay down their arms by 1 p.m. on Sunday, local time, saying that the lives of those who accept the demand will be spared.

But the Ukrainian side did not surrender by the deadline.

A commander of the Azov battalion that is defending the city told NHK on Sunday that they will never surrender.

Maxim Zhorin said fighters in the suburbs of Kyiv are now heading to Mariupol.

Police in Mariupol told a local TV station that many women and children are sheltering from the shelling inside the steelworks.

Russia's state-run media quoted the head of the country's forces in the city as saying they are ready to intensify attacks after their proposal was rejected.

Before that, Russian Defense Ministry spokesperson Igor Konashenkov argued that the Ukrainian forces include foreign fighters.

He warned that Ukrainians who continue to resist will be destroyed.

Speaking to US broadcaster CBS on Sunday, Ukrainian Foreign Minister Dmytro Kuleba said "Mariupol may be a red line."

He stressed that if the death toll among the forces defending the city rises further, ceasefire negotiations could be suspended.