ROBERT SIEGEL: The head of the United Nations has harsh words for whoever carried out an attack on Syrian University students, as they were taking exams. Two explosions at the university in Aleppo killed more than 80 people yesterday and injured some 200. Today, Secretary-General Ban Ki-moon said such heinous attacks are unacceptable. And he added that deliberate targeting of civilians constitutes a war crime. But who carried out that attack is very much in question.

We're joined now from Aleppo by Martin Chulov of the British newspaper the Guardian. And first, you report today that after the university explosions, they were followed today by a government defensive in the city. What's going on?

MARTIN CHULOV: The government was trying to push towards the east from the west, they were pushing into rebel-held territory, planning that this was direct reprisal for what had taken place at the university yesterday.

We are in Aleppo. We heard those explosions yesterday afternoon. We knew something significant had taken place. But we weren't able to establish what it was until students started to stream out of the university and tell their stories to us last night and this morning.

: Well, from what you've been able to glean from their stories and from any other evidence that you've had access to, what happened at the university? Do we know who was responsible for those two explosions?

CHULOV: You know, we don't at this point. We do know that two explosions struck buildings on the western side of the campus. And that's significant for two reasons. First, it does tend to suggest that whatever it was that hit the building came from west of the city. Now, west of the city is contested territory. It is partly held by opposition groups, some jihadi elements amongst them, and there's also a regime presence there.

Students who did come to talk to us did say that they were just about to start to sit exams when the first explosion hit. A second explosion hit the building three minutes later. Now, they are claiming that rockets hit their campus. Some other students that we did speak to said that they'd heard planes above the campus. And some their colleagues said that that just wasn't the case. And suspicion really is affirming now on rockets being fired from west of the city. By who, we don't yet know.

: Now, as I mentioned, Ban Ki-moon reminded all that deliberate targeting of civilians is a war crime. Is it clear that this was a deliberate attack on the university? Or, as I read, there's a building adjacent to the university that might also have been a target here.

CHULOV: The suspicion is that the university was deliberately targeted. It's in the part of the city which is firmly controlled by the regime. Even after the seven-month siege of east of the city, it's very difficult to anybody with hostile intention to get into anywhere near the university and cause any trouble there. Whoever fired those rockets was likely intending to hit the campus.

: The place of this attack - a university - the number of dead, the number of injured seems shocking. I wonder, are people in Aleppo capable of any more shock given what they have already experienced in this war or did this have any special import for them?

CHULOV: It has resonated. You know, the city had been abandoned. It had a foreboding sense for many, many months now. And I guess a relative normalcy had returned in the past week or so, when there had been this infusion of fresh food for the first time.

But today, things were different. People were talking about what had taken place and flagging this as a potential tipping point, which really could chip away at whatever it is that has taken place in the city and certainly shape the narrative of the battle one way or another.

: Martin Chulov, thank you very much for talking with us.

CHULOV: You're welcome.

: Martin Chulov covers the Middle East for the Guardian and he spoke to us from the city of Aleppo in Syria. Transcript provided by NPR, Copyright NPR.