JUDY WOODRUFF: Tomorrow marks five years since Syria's brutal civil war began, killing hundreds of thousands and displacing millions more.

The humanitarian situation remains the most dire since the Second World War, as halting peace talks toward a resolution begin again.

Chief foreign affairs correspondent Margaret Warner begins our coverage.

MARGARET WARNER: Huddled together, hand in hand, hundreds of migrants streamed out of a camp in Northern Greece today. They trudged along muddy trails and forded surging rivers in search of a break in the border fence.

MAN: We hope we can cross because we are a lot of people. Our number is big now.

MARGARET WARNER: They're fleeing the overcrowded, rain-soaked Idomeni camp, where they have been stranded since its northern neighbor Macedonia closed its border to refugees last week. Many are Syrians fleeing the war in at home.

In Geneva today, the first round of United Nations-brokered peace talks aimed at ending that conflict got under way, as U.N. special envoy to Syria, Staffan de Mistura, hosted Syria's ambassador to the U.N.

STAFFAN DE MISTURA, UN Special Envoy for Syria: We believe that we should have at least a clear road map. I'm not saying an agreement, but a clear road map, because that's what Syria is expecting from all of us.

As far as I know, the only plan B available is return to war, and to even worse war than we had so far.

MARGARET WARNER: The start of talks come as a tenuous cessation of hostilities in Syria entered its third week.

Today, five years since the conflict began, UNICEF painted a dire picture for the country's children. One in three Syrian children has been born since the war broke out. And 8.4 million, or nearly 80 percent of Syrian children, are affected by the violence, either within the country or as refugees elsewhere.

Meanwhile, the U.N. Refugee Agency, UNHCR, reported that 46 percent of the more than 150,000 refugees inundating Europe this year came from Syria. Those numbers outpace last year's, when a record one million people fled to Europe.

Many make the perilous sea crossing from Turkey to Greece, but now, because Balkan borders are blocked, thousands are stranded in Greece. At an E.U. summit last week, Turkish Prime Minister Ahmet Davutoglu offered to take back huge numbers of migrants in exchange for $3 billion in aid and progress on Turkey's joining the European Union.

The E.U. summit with Turkey resumes later this week, but the U.N. Refugee Agency has strongly objected to the deal, saying it would violate the European Human Rights Convention.

That's where Jeffrey Brown picks up the story.

JEFFREY BROWN: And joining me is the new head of that agency, Filippo Grandi, United Nations high commissioner for refugees.

Welcome to you.

FILIPPO GRANDI, UN High Commissioner for Refugees: Thank you.

JEFFREY BROWN: Let's start with that tentative deal that Margaret was just talking about between the E.U. and Turkey. What is your objection to having Turkey bring back some of those refugees?

FILIPPO GRANDI: I think we should put this in context.

Turkey is the country that hosts the largest number of Syrian refugees. We have heard it 2.7 million. In fact, it's the countries that host the largest numbers number of refugees worldwide, all kinds of refugees.

And we have always promoted the idea that the responsibility of hosting Syrian refugees should be shared more widely. Many of them have reached Europe together with refugees of other countries, and we have always encouraged Europe to manage that flow in an orderly way.

Unfortunately, this wasn't done. Chaos has followed. What we see in Greece today is very worried. Europe is reacting in an emergency manner and is now proposing to — is now discussing with Turkey the possibility of sending back to Turkey some of these people. Now, what we're seeing…

JEFFREY BROWN: Would that not create more of an order — orderly passage?

FILIPPO GRANDI: Well, first of all, it needs to be done, and if it is done, it is called technically readmission of refugees from one country to the other.

It needs to be done in a manner that respects the human rights, the rights of these refugees. So, what we're saying to Europe, what we're advising Europe, if it is done, it needs to be done in a manner that fully, fully ensures the guarantees for the people that go back that their protection and their rights are observed.

JEFFREY BROWN: Speaking of Turkey and other countries that border Syria, where there are so many refugees, and so many financial precious on these countries — now, you have had a lot of pledges of money. Do they get — do they have the resources to handle the refugees they have? Or is the money coming through that you need?

FILIPPO GRANDI: There was a conference in London at the beginning of February in which $11 billion were pledged to support refugees and hosting communities and host countries in the region.

Some of that money has been paid and — pledged and paid, but we haven't got the full picture yet. What we are telling donor governments is that those pledges need to be expedited, because, for people on the ground, results have to be tangible and visible. Otherwise, the risk is that they will move on and also try to reach Europe.

JEFFREY BROWN: When you look at countries in Europe closing their borders, you look at political trends, even this weekend elections in Germany, do you see the tide turning against refugees in Europe?

FILIPPO GRANDI: We see a hardening of the position of certain governments in respect of receiving refugees.

What we are, of course, telling Europe is that the responsibility to take care of refugees should be shared globally. This is — Syria has shown that this cannot be any more the responsibility of two, three countries bordering the country at war, Syria in this case, but has to be shared more widely, including by Europe.

JEFFREY BROWN: So, thousands of refugees now stranded in Greece, right? What should be done for those? What is the situation there? How dire is it? And what should be done for those people in the short term?

FILIPPO GRANDI: The situation is dire.

People were trying to move on for Greece. The northern border was closed, so now about 40,000 are stranded there, and more are coming. The important thing is to find adequate sites to host them temporarily until solutions are found, including, perhaps, relocation through Europe, a decision that Europe made many months ago, but which it didn't implement.

JEFFREY BROWN: Well, what do you — you were talking earlier about donor countries, the pledges you have received. I wonder about the United States. What do you want to see come from the United States? Is it more money? Should the United States take in more refugees?

FILIPPO GRANDI: The United States is the largest donor country to refugee programs.

And it's also the country that takes the largest number of resettled refugees from old nationalities. Of course, the number of Syrians that all countries are taking through resettlement~ and other legal means of transferring them from one country to the other, of course, those numbers are still inadequate, compared to the 4.7 million that are hosted by the countries neighboring Syria.

JEFFREY BROWN: So, why is this so tough to crack? I mean, why does it seem to get worse? Why is there intransigence from governments that keeps people in this flight? And how long do you think the world has?

FILIPPO GRANDI: I think there's a mixture of motives that have made governments and sometimes public opinion more resistant to hosting refugees.

There are economic reasons, economic downturn. There's security reasons, the fear, the unjustified fears that refugees bring terror. Refugees flee terror. They don't bring terror to countries. Their arrival is very carefully vetted, so there should be no fear.

But there is an irrational fear, which is, in many countries, fueled by political propaganda.

JEFFREY BROWN: And Margaret's piece was talking about the potential prospects for a peace settlement, but who knows, right?

Do you expect, for the time being, that the flight of refugees will continue?

FILIPPO GRANDI: Until there is war, people will try to flee.

This is in human nature. People are afraid of bombs, of destruction and want to go away. That is why what has happened in Geneva today, five years after the beginning of the war, the resumption of peace talks, is so important. It must succeed.

JEFFREY BROWN: Filippo Grandi is U.N. high commissioner for refugees.

Thank you so much.

FILIPPO GRANDI: Thank you.