The world wants to help Ukraine. But organizations delivering humanitarian aid face major obstacles.

“It’s just incredibly difficult to deliver any kind of response safely when the fighting is as horrendous and constant,” said Dan Stewart, a spokesman for Save the Children, which provides emergency aid to children in conflict zones.

Hundreds of thousands of people in Ukraine lack food, water, heat, electricity and medical care, according to the International Committee of the Red Cross. Temperatures dip below freezing at night, and Save the Children has received reports of children becoming ill from the cold. In Mariupol, a besieged city in Ukraine’s southeast, locals are reportedly melting snow to drink.

Ukrainian authorities have accused Russia of shelling humanitarian corridors, in violation of temporary cease-fire agreements. Russian attacks have destroyed vital infrastructure, and the port of Odessa — where almost all of Ukraine’s imports by sea arrive — is closed because of the war.

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The Kyiv City Charity Foundation Food Bank, which has distributed food to people in need in Ukraine’s capital since 2011, has been “operating more actively than ever,” with an “army” of volunteers who distribute food even under Russian fire, Nataliia Radetska, the food bank’s deputy board head, said in an email to The Washington Post on Monday.

But the food bank has been short on food since Ukrainian plants halted operations, she said.

“That’s why food aid from abroad is so important for us,” Radetska wrote.

Ukrainian truckers load up with donated supplies before going back to fight

International organizations such as Save the Children and the U.N. World Food Program are surging staff and diverting resources to Ukraine and neighboring countries. Events on the ground are changing so rapidly, though, that it can be difficult to anticipate where supplies will be needed, Stewart said. And aid groups often have to build partnerships with locals from scratch.

Some had smaller-scale projects in parts of Ukraine before the war, and existing relationships with local groups have allowed them to stock supplies in Ukrainian towns before Russian troops and missiles arrive.

“We’re really in a race against time to preposition food in areas where fighting is expected to flare up,” said Steve Taravella, senior spokesman for the World Food Program.

Vice President Harris, visiting Poland on Thursday, announced the United States would donate $50 million to the WFP.

The organization plans to assist more than 3 million people inside and outside of Ukraine in the coming months, he said. It has set up operational bases in three locations — Lviv in western Ukraine and two others that Taravella declined to name, citing security reasons. The hubs will supply and organize humanitarian convoys into conflict areas.

WFP is also establishing hubs in Poland to deliver food into Ukraine. But “where possible, we’re trying to purchase food from within Ukraine, which will help keep their economy going,” Taravella said.

WFP has begun distributing bread in several cities, including Kharkiv, and dispatched emergency food assistance to the capital. But Taravella emphasized that these are Band-Aid solutions.

And with able-bodied Ukrainian men joining the fight, finding drivers to move supplies has proved challenging, David Beasley, executive director of the World Food Program, told CNN this week.

The International Committee of the Red Cross is also building up its supply routes to bring medical aid, food, water and hygiene items into Ukraine.

The Red Cross said it had delivered supplies to two hospitals in Mariupol and provided medical assistance in centers for displaced people in the city. The organization partnered with Doctors Without Borders to send surgical materials to three main hospitals in Kyiv, and sent enough insulin to the Odessa and Dnipropetrovsk regions to cover thousands of people.

The Ukrainian government, meanwhile, collected and delivered more than 400 metric tons of medicines and medical supplies — mostly from European donor countries and United Nations agencies — during the first 11 days of the war, according to the latest update from the health ministry.

Still, vast needs remain — as do challenges in delivering aid to civilians bearing the brunt of the Russian attacks.

Materials to treat traumatic injuries — including surgical supplies and trauma and mass-casualty kits — are in high demand, said Tim Shenk, a spokesman for Doctors Without Borders. Carla Melki, the organization’s emergency coordinator in Odessa, said drugs and items for treating chronic illnesses are also in short supply across Ukraine.

The Ukrainian health ministry set up a hotline and a Google Form to collect donations. California-based nonprofit Nova Ukraine is chartering planes to ship medical supplies from the United States to Poland, co-director Igor Markov said.

Evacuations from some encircled cities resumed Thursday. But in Mariupol, which continues to face heavy Russian bombardment, shelling has prevented civilians from leaving. The one functional hospital was at capacity, an adviser to the mayor’s office said Thursday. The northern city of Chernihiv also appears to have been isolated by Russian troops, the Pentagon said Thursday.

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The Red Cross delivered medical supplies to health facilities and food, water and hygiene items to shelters in Mariupol last week, said Jason Straziuso, an ICRC spokesman.

“But we haven’t been able to bring in new supplies since,” Straziuso said. “So we’ve been calling on all sides to allow humanitarian organizations like ours to be able to safely carry out a resupply.”

On Wednesday, a Russian airstrike struck a maternity hospital in the city, killing at least three people. The attack alarmed “the whole humanitarian community,” said Melki, of Doctors Without Borders. “If we know that hospitals will not be bombed, it would be easier to work.”

International aid groups are well versed at operating in conflict zones. But some have assessed that the dangers in Mariupol and other front line cities are too great for staff to deliver aid. Teams from the Red Cross had a close call on Sunday, when they were exposed “at close range” to fighting as they tried to help facilitate safe passage out of the city, Straziuso said.

About 90 percent of the 600 Red Cross staff members in the country are Ukrainian. With families and homes under threat, many are personally affected by the conflict, “so operating in these situations becomes very challenging,” Straziuso said, “from a human perspective.”

The Red Cross has been in talks with Ukrainian and Russian authorities, pushing for agreements that “specify location, time and duration” of humanitarian corridors — and whether assistance could be brought into the city when those routes are open, Straziuso said.

Straziuso declined to comment on whether Russian officials showed willingness to allow the resupply of aid to Mariupol.

High-level talks between Ukraine and Russia in Turkey failed to produce an agreement on Thursday. Ukrainian foreign minister Dmytro Kuleba said Ukraine had received no response to its proposals for a 24-hour cease-fire and humanitarian relief for Mariupol.

Observers expect the humanitarian situation to worsen as the conflict grinds on and Russian President Vladimir Putin appears to resort increasingly to deadlier tactics.

“The only way to really guarantee full humanitarian access and aid delivery would be complete cessation of hostilities,” said Stewart, from Save the Children.