Russia and Ukraine have blamed each other for a failure to provide safe passage to civilians fleeing two cities besieged and bombarded by Russian forces, on the 10th day of a war that has fuelled Europe's biggest humanitarian disaster in decades.
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The war, which began with Russia's invasion on February 24, has sent nearly 1.5 million refugees fleeing westward into the European Union and provoked unprecedented international sanctions on Russia and warnings of a global recession.
The Russian defence ministry said its units had opened humanitarian corridors near the cities of Mariupol and Volnovakha, which have been encircled by its troops.
But in Mariupol, the city council said Russia was not observing the ceasefire and it asked residents to return to shelters and await further information.
Russia's defence ministry accused Ukrainian "nationalists" of preventing civilians from leaving.
The International Committee of the Red Cross said it understood that relocation of civilians from Mariupol and Volnovakha would not start on Saturday because of the conflict.
"We remain in dialogue with the parties about the safe passage of civilians from different cities affected by the conflict," the ICRC said.
"The scenes in Mariupol and in other cities today are heart-breaking. Any initiative from the parties that gives civilians a respite from the violence and allows them to voluntarily leave for safer areas is welcome."
The southeastern port has endured heavy bombardment, a sign of its strategic value to Russia due to its position between separatist-held eastern Ukraine and the Black Sea Crimean peninsula, which Russia seized from Ukraine in 2014.
"This night the shelling was harder and closer," a staff member from Doctors without Borders/Medecins sans Frontieres said.
There was still no power, water, heating or mobile phone links and food was scarce.
The Ukrainian government said the plan was to let about 200,000 people leave Mariupol and 15,000 to depart Volnovakha.
Only 17 people left Mariupol on Saturday and no one had left Volnovakha, Tass cited pro-Russian separatists as saying.
Despite the limited ceasefire plans, the Russian defence ministry said a broad offensive would continue in Ukraine, where it denies targeting civilians or invading, calling its actions a "special military operation".
Russian forces were carrying out strikes on military infrastructure and forces from separatist-held Donetsk were tightening the encirclement of Mariupol, Defence Ministry spokesman Igor Konashenkov said.
Aid agencies have warned of a humanitarian disaster across the country.
The number of refugees could rise to 1.5 million by the end of the weekend from a current 1.3 million, the head of the United Nations refugee agency said on Saturday.
Women and small children crossed at the Medyka checkpoint in southeastern Poland in freezing conditions.
A man crossing the other way yelled at the crowd that men should return to Ukraine and fight.
One woman, struggling to carry half a dozen bags, wept when the snacks she had packed for her and her young son, who was clutching a green dinosaur toy, fell to the ground.
She gave the boy a bag to carry as they trudged slowly on.
President Vladimir Putin's decision to invade has drawn worldwide condemnation.
Officials in Ukraine have reported thousands of dead and wounded civilians.
Russia says its aim is to disarm its neighbour, counter what it views as NATO aggression and capture leaders it calls neo-Nazis.
On Saturday it accused the US and its allies of acting like a bandit and threatened to retaliate without giving details.
"As you understand, there must be a corresponding response to economic banditry," Kremlin spokesman Dmitry Peskov said.
The conflict has also shaken international diplomacy over Iran's nuclear program, one of the few areas where Russia and the United States had been working together to curb what the US suspects is an Iranian plan to develop nuclear arms.
Russian Foreign Minister Sergei Lavrov said on Saturday the new sanctions imposed on his country had become a stumbling block for clinching a nuclear deal with Iran.
A senior Iranian official told Reuters the Russian stance was not helpful.
Ukraine says Russian forces have focused efforts on encircling Kyiv and Kharkiv while aiming to establish a land bridge to Crimea.
Australian Associated Press