The United Nations needs safe passage to deliver humanitarian aid to conflict zones in Ukraine, a senior official with the organization told the Security Council on Monday.

"Civilians in places like Mariupol, Kharkiv, Melitopol and elsewhere desperately need aid, especially life-saving medical supplies," undersecretary-general for humanitarian affairs Martin Griffiths told an emergency meeting on the disaster sparked by Russia's invasion.

Griffiths urged all sides to ensure that civilians, homes and infrastructure in Ukraine were safeguarded. 

"This includes allowing safe passage for civilians to leave areas of active hostilities on a voluntary basis, in the direction they choose," he said, after Ukraine rejected an earlier deal that would only allow its civilians to evacuate into Russia or Belarus.

The meeting came as Ukraine and Russia seek an agreement on creating "humanitarian corridors" out of pummeled cities, as the civilian toll from the Russian assault mounts.

Russia said Monday it would open humanitarian corridors on Tuesday from 0700 GMT, subject to Ukraine's approval, listing evacuation routes from Kyiv as well as Mariupol, Kharkiv and Sumy -- all of which have been under heavy Russian attack.

Kyiv's UN ambassador, Sergiy Kyslytsya, said the conflict had blocked exports through Ukraine's seaports, cutting off vast quantities of agricultural goods vital to the world's food supplies.

"The implications at the global level will be catastrophic," said Kyslytsya, noting that his country produces 55 percent of the world's sunflower oil.

He also warned of the health threat from so many dead lying uncollected on the battlefield.

"We are speaking about dozens of thousands of bodies decomposing in the fields of Ukraine, I'm talking about the bodies of the Russian soldiers," he said, calling on the International Committee of the Red Cross to help repatriate fallen combatants and prisoners of war.

- 'Mercenaries' -

Calling for restraint on all sides, China's ambassador said an already dire situation could only be made worse by shipments of arms to Ukraine, as well as the deployment of "mercenaries."

The Pentagon said Monday that Russia is recruiting Syrians and other foreign fighters as it ramps up its assault on Ukraine. 

Tens of thousands of foreign volunteers have also traveled to Ukraine to fight with Kyiv's forces, according to the foreign minister.

Russia's ambassador to the UN, Vassily Nebenzia, said there was "information warfare" underway and slammed the ban by Western countries on Russian state media outlets.

Griffiths said the UN also urgently needs a system of "constant communication" with all sides, adding it had sent a team to Moscow for a first technical meeting at the Russian defense ministry.

The goal, he said, is to work on better humanitarian civil-military coordination to be able to "scale up" UN operations. 

A senior UN representative, speaking on condition of anonymity, said officials also hoped to ensure that humanitarian convoys were not targeted by Russian attacks. 

To date, the UN has had no involvement in the establishment of humanitarian corridors.

Addressing the Council, Linda Thomas-Greenfield, Washington's ambassador to the UN, slammed Russia's attack on Ukraine.

"Dozens of children have been killed in (Russian President Vladimir) Putin's war, she said, noting that "actual numbers are likely far greater."

Thomas-Greenfield warned that children were being "severely traumatized" by the violence and destruction to the point of no longer speaking, and that the physical and psychological wounds of the conflict would be long-lasting.  

"It's clear Mr. Putin has a plan to brutalize Ukraine," she said.