JUDY WOODRUFF: Last Friday, the U.S. and Russia announced a deal to resume a tenuous cease-fire in Syria, in order to get much needed humanitarian aid to badly deprived civilians throughout the country. It went into effect Monday evening, and, thus far, the results are mixed.

Chief foreign affairs correspondent Margaret Warner begins our coverage.

MARGARET WARNER: The day began with claims of progress in the four-day old cease-fire. Syrian state television showed bulldozers apparently clearing the Castello Road into war-ravaged Aleppo.

That's the crucial link for humanitarian aid convoys to enter Aleppo from Turkey. The Russians also announced that Syrian army units had pulled out of the area. But, within hours, all that changed.

LT. GEN. VLADIMIR SAVCHENKO, Russian Reconciliation Center in Syria (through translator): Opposition groups have not started pulling armor and weapons from Castello Road. As a result, Syrian military vehicles and government troops were returned to the positions they occupied earlier.

MARGARET WARNER: Rebel fighters denied that, and said government forces never left in the first place.

FAYZ, Free Syrian Army Fighter (through translator): We are currently around a mile away from Castello Road, whereas the regime can be seen over there traveling through normally. Nothing has changed for them. It's only us who have been forced to retreat.

MARGARET WARNER: The finger-pointing left U.N. convoys stuck at the Turkish border for another day, without Syrian government permission to enter.

JENS LAERKE, UN Office for the Coordination of Humanitarian Affairs: We are as ready to go as we can possibly be. The modalities for ensuring safe passage have not yet been cleared and given to us so that we can move.

MARGARET WARNER: Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov had negotiated the cease-fire in Geneva last weekend. They spoke by phone today, and, according to the State Department, Kerry said the delays on opening access for humanitarian aid were unacceptable.

The White House went further, blaming the Russians.

JOSH EARNEST, White House Press Secretary: They are the ones that have made a commitment to use their influence with the Assad regime to reduce the violence and allow humanitarian access. And either the Russians are unable to live up to the agreement. Maybe they don't have the juice and influence that they claim to have and that we all thought they had. Or maybe they're just unwilling.

MARGARET WARNER: Meanwhile, there are other signs the cease-fire is fraying. Heavy fighting was reported today around the capital, Damascus. And tensions have cropped up within the U.S.-backed coalition battling the Islamic State in Syria.

This amateur video today was said to show Americans, possibly special forces, leaving a Syrian border town after rebels protested their presence.

JUDY WOODRUFF: And Margaret joins me now.

So, Margaret, one week in, it looks like this deal is not holding up well. What has gone wrong, and has anything going right?

MARGARET WARNER: Well, the one thing that's going right, Judy, is you don't have all this heavy bombing of civilians, so we can't lose sight of that. People are allowed to go out in Aleppo and shop if there's anything to buy.

But what's not happening, as we just showed in the setup, is that humanitarian aid is not getting through. And that was, as Josh Earnest said, the Russians' job to make sure that President Assad would let these convoys in, and they're just not getting in.

The two tracks, if both of those were satisfied, then they were going to set up this joint operations center, where the U.S. and Russians would somehow work together to target the Islamic State and the al-Qaida-linked group which used to be called the Nusra Front. But the White House made clear today, unless the other two checks are satisfied, they're not going to even get to this joint integration center or operational center.

JUDY WOODRUFF: Now, the Russians are saying, though, that the United States bears some of the responsibility.

MARGARET WARNER: Well, you know, they have a point.

The U.S. responsibility was to lean on its clients, which are the U.S.-vetted or CIA-vetted rebels, the so-called moderate opposition. And they have been, in many areas, really entangled with this al-Qaida-linked group, not ideologically, but simply because they fight more effectively together.

And I have been told by people who have spoken to the military commanders of these groups that many of them are very suspicious. They say, well, if we have to disentangle from the Nusra fighters, who are the most effective ones, means we have to fall back, the U.S. and Russians will bomb the Nusra fighters, and Assad's forces will come take the territory.

JUDY WOODRUFF: So, Margaret, explain, though, this apparent division between the State Department and the Pentagon over working with the Russians.

MARGARET WARNER: Well, it's really interesting, Judy, because not that you don't always have this kind of friction — or often — but to have it burst into the open like this.

The other day, I was at a conference in which the new head of CENTCOM, General Votel, was speaking, and he said, look, we can't — there's a trust deficit with the Russians. We don't know what their objectives are. They will say one thing, and they don't follow up. And this is particularly true in this whole Ukraine over these years of the Ukraine conflict, where the Russians would promise to do one thing and they would do something else.

There are also specific risks. They are not going to really operate together. They were supposed to share intelligence. So, here are the Nusra fighters, here are the ISIS fighters.

But what the Americans are very afraid of is that the Russians will take the information that superior U.S. intelligence has, either use that information to bomb the U.S.-backed rebels or pass that info on to the Assad forces. One military colonel, one Army colonel suggested that to me.

And that also it's hard to tell the Russians what we know without indicating how we know it. So the secretary of defense, Ash Carter, has actually spoken out.

And the other interesting I heard today, Judy, is that this debate is being used by opponents of this deal in Moscow to say, well, if the American military isn't even going to obey the orders, why should we do anything?

JUDY WOODRUFF: Is this really, though, Margaret, the last gasp of this information on Syria, and if that's the case, then what happens after that?

MARGARET WARNER: I'm afraid so.

I went to the very first — Secretary Kerry keeps saying, we want to get to this political transition, these talks where there will be a discussion of a process and President Assad will slowly fade away.

But I went to the very first talks back in, you know, Switzerland almost three years ago. I don't think it's moved at all. And at this point, that is a distant dream. If they could only get at least an end to a lot of the carnage on the ground — in other words, they — Kerry and Lavrov are now negotiating over a fifth of a loaf, and that now even appears in doubt.

JUDY WOODRUFF: Tough, tough story to follow.

MARGARET WARNER: Yes, very.

JUDY WOODRUFF: Margaret Warner, we thank you.