HARI SREENIVASAN: In Syria, where a five-day-old cease fire brokered by Russia and the United States is on shaky ground. Russia claimed today U.S.-led air strikes on a government military base surrounded by Islamic State militants killed at least 62 Syrian soldiers.

In a written statement, U.S. Central Command acknowledged it halted the air strike in eastern Syria, near the ISIS stronghold of Raqqa, immediately after being told by Russian officials that Syrian forces were being targeted by accident. CentCom said its coalition forces, quote, "would not intentionally strike a known Syrian military unit."

Meanwhile, convoys of aid supplies for Aleppo and other besieged areas remain stalled, with United Nations officials blaming the Syrian government for blocking access.

For more on the situation, I'm joined via Skype from Beirut by "Washington Post" correspondent Liz Sly.

Liz, given the information that's coming out this afternoon, what does this do to the truce that's been in place for the last week?

LIZ SLY: Well, it doesn't do very much for it at all. We know that the truce was already not going very well at all. Lot of things that were supposed to happen, didn't happen, such as delivery of aid to needy civilians. There has been lots of fighting along the front lines.

And now, we have this allegation that the Americans have bombed and killed 60 Syrian soldiers, and they're kind of admitting that they probably did do it. So, this is going to send tensions sky high and make things only much more complicated than they already were.

HARI SREENIVASAN: At this point, does President Bashar al-Assad, accept the U.S.' contention that this was an accident, that this inadvertent, or does he go forward and say, this is an act of war?

LIZ SLY: Well, I'm not sure that President Assad was very keen on this truce to begin with. I don't think he was too keen on the Americans and Russians teaming up to do military activity in Syria at all. So, this definitely bolsters his position that this was not a good truce, this is not a good cease-fire, it's not a good time for ceasefire.

Now, the Russians are taking a different tact. The Russians are saying this proves that the Americans have to coordinate with us.

Now, the Americans do say they did coordinate with the Russians. They said they did happen to tell the Russians that they did plan to bomb this position, and that the Russians knew they were going to bomb this position. But — so we don't really know what happened here, but whatever it was, it's a giant mess.

HARI SREENIVASAN: You mentioned this earlier — the humanitarian aid that was supposed to get through. That was one of first phases of the truce. What was the — what was the progress on that this week?

LIZ SLY: There's been no progress at all. The aid hasn't gone into any of the communities at all because the Syrian government has refused to give permission for it to cross the border to enter the city it's supposed to enter.

HARI SREENIVASAN: And there was also supposed to be military cooperation by the U.S. and the Russians. Unfortunately, we're seeing that that was not the case, that these two clearly hadn't coordinated, at least in the site of this incident this afternoon.

LIZ SLY: Well, the military coordination was supposed to follow seven days of calm during which aid flowed unimpeded to the neediest civilians. Now, there has been relative calm, although fighting has continued on the front lines, but the aid has not flowed. So Americans are saying the truce hasn't worked yet. The Russians are accusing the Americans of not having disentangled the fighters they wanted to disentangle, and now as we see we have this new situation where it does look like some kind of ghastly accident, or whatever you want to call it — act of war– which is what the other side will almost certainly say — has occurred to just blow it all up before it really began.

HARI SREENIVASAN: "Washington Post" correspondent Liz Sly joining us from Beirut tonight — thanks so much.

LIZ SLY: Thank you.