moderate rebel
HOUSE OF REPRESENTATIVES, COMMITTEE ON FOREIGN AFFAIRS, HEARING - U.S. POLICY AFTER RUSSIA'S ESCALATION IN SYRIA, Assistant Secretary Bureau of Near Eastern Affairs U.S. Department of State, WEDNESDAY, NOVEMBER 4, 2015, Transcript (pdf)
Ambassador PATTERSON: That is true in the north, Mr. Issa. I would say that it is not true in the south. But, certainly, in the north, the al-Nusra Front, which is an al-Qaeda affiliate, has absorbed, as have other smaller groups, have absorbed a number of what we would have previously called the moderate opposition, yes, that is correct.



Comment: Apparently the U.S. thinks it can have it both ways. First, they want to be able to claim their rebels are moderate, in contrast to al-Nusra. Second, they want to be able to criticize Russia for bombing their "moderate opposition", who are embedded with al-Nusra! Russia is perfectly legally justified in bombing Nusra positions in Aleppo, just as the Syrians are justified in attacking and attempting to liberate the areas occupied by the al-Qaeda affiliate. If it walks like a duck, quacks like one, and fights alongside a whole army of them, it's a duck.
isis ducks



U.S. Department of State, Mark C. Toner Deputy Spokesperson, Daily Press Briefing, Washington, DC, February 22, 2016
QUESTION: -- [...] Is there not - I mean, my understanding is that at least some of these [moderate] groups are commingled [with al-Qaeda]. You assert that you feel that you are able to delineate the territories that are inhabited by ISIL or by Nusrah or by the other groups deemed terrorist. What makes you so confident you can do that? Do you not believe that there is at least some commingling of these groups? Or intermingling?

MR TONER: I would respectfully defer to others with a better knowledge of conditions on the ground. But yes, I believe there is some commingling of these groups. I think that's a reality; we've talked about it before.
Department of Defense Press Briefing by Col. Warren via Teleconference from Baghdad, Iraq
Colonel Steve Warren, Operation Inherent Resolve spokesman - April 20, 2016
COL. WARREN: [...] What I do know is that we have seen, you know, regime forces with some Russian support as well begin to mass and concentrate combat power around Aleppo. So this is something we're concerned about and something we'll keep an eye on.

That said, it's primarily al-Nusra who holds Aleppo, and of course, al-Nusra is not part of the cessation of hostilities.
Russian Military Buildup Near Aleppo, Syria, Threatens Truce, Kerry Warns - NY Times April 23 2015
Mr. Kerry said that the Russians might be moving on Aleppo because members of the Al Nusra Front, an affiliate of Al Qaeda, were mixed throughout parts of the region, and that they were terrorists not party to the cease-fire. At the same time, he said, the region is home to insurgent groups that oppose Mr. Assad and have agreed to the cease-fire.
"That has proven harder to separate them than we thought," Mr. Kerry said. "And there's a Russian impatience and a regime impatience with the terrorists who are behaving like terrorists and laying siege to places on their side and killing people."
For the last several days the government held parts of Aleppo city and the 2,000,000 inhabitants and refugees there have been under constant bombardment with improvised gas-canister mortars and rockets from the al-Nusra side.

Some heavy operations against the al-Qaeda held areas in Aleppo governate and Aleppo city are inevitable and now seem to be imminent. The statements the U.S. officials made above seem to justify such an operation.