The long running civil war in Syria appears to be entering a new and even grimmer stage, with troops loyal to President Bashar Al-Assad intensifying a widespread offensive on the city of Aleppo and elsewhere, including, according to residents, renewed use of chlorine barrel bombs.

Opposition forces were fighting desperately to break a now solidly-established siege of eastern Aleppo, the largest remaining center of resistance to the Russian-backed forces of Bashar Al-Assad, where food, fuel and medical supplies for some 350,000 people are dwindling.

An opposition offensive that began earlier in the week was renewed Friday morning, according to humanitarian workers in the city who were contacted by Fox News, though they could offer no sense of how the battle was going.

Meantime, regime forces were on an offensive of their own, not only in Aleppo but at besieged areas throughout the devastated country, with strong Russian air support.

“There is a regime offensive across the board,” said Valerie Szybala, executive director of the Syria Institute, a non-partisan think tank studying the conflict. “It undermines the possibility of getting back to the [negotiating] table.”

It has also created a de facto blockade against humanitarian relief efforts that U.N. officials say they are ready to deliver, if only given a chance.

“We are ready, able and willing to go, if there are pauses in the fighting,” declared Jan Egeland, a Norwegian politician acting as special advisor to the U.N. Special Envoy for Syria, on Thursday.

Except the pauses aren’t happening. “There is fighting in too many places,” Egeland said. As a result, humanitarian relief—he did not say how much—had reached only 40 per cent of some 1.2 million people the U.N. hoped to reach in July.

Earlier U.N. –sponsored deliveries to besieged areas were sparse, sporadic and often limited by the Assad regime to supplies that the government forces decided were acceptable, which often excluded vital medical supplies, for example.

A key factor in the battle is incessant Russian air strikes, which have, according to military observers, continued at a high pitch, even as Russian officials remain involved in international ceasefire talks and claim to be holding open “humanitarian corridors” to allow Aleppo residents to escape from bombing and artillery barrages.

“We have seen Russian airstrikes go up to pre-cessation-of-hostilities levels, and not only in Aleppo City,” said Genevieve Casagrande, an analyst at the Washington-based Institute for the Study of War, referring to a watery cease-fire agreement negotiated last February, which began to erode almost immediately.

“They are targeting neighborhoods where everyone knows there are only civilians,” said Hamza Al Khateeb, director of the Al Quds hospital in Aleppo, who spoke with Fox News by telephone. He said opposition forces were able to identify Russian MiG warplanes not only by sight but by air chatter as they they made bombing runs.

The Russian attacks have often been aimed at civilian targets, including hospitals, clinics and other health facilities—including Al Quds, which was attacked seven times in April, according to Al Khateeb. More recently, two improvised barrel bombs—used by Assad regime forces—had landed across the street, no more than 50 ft. from the battered hospital.

Air attacks had also targeted morgues, he said, killing at least two medical workers and injuring five.

Another doctor was reported injured by Dr. Osama Abo El Ezz, General Surgeon and Aleppo Coordinator of the Syrian American Medical Society, who had to break off a phone call with Fox News to attend to a 15-year-old severely injured in the chest and abdomen by bombing. The teenager died.

( All told, 44 health facilities were reportedly hit in June alone, according to the U.N.’s Egeland, who said “we are investigating all of these.”)

Abo El Ezz called the general situation “horrific,” and said medical workers were treating as many as 100 people on some days, with 20 of them requiring work in intensive care units. “I think the Russians will do their best to kill more and more to make the opposition break,” he said.

Abo El Ezz also made the assertion that chlorine bombs had been dropped “on the north side of Aleppo City.”

“We are running out of many things,” said Al Quds director Al Khateeb. “We have medical supplies for about one month,” echoed Abo El Ezz. Electricity at their hospitals is provided by generators, which run for only part of the day, and fuel supplies for them are running low.

Other items in short supply included purified water and just about any kind of fresh food, as well as milk powder for children.

As for the so-called “humanitarian corridors,” Aleppo residents considered them a fraud. They remain under Russian control and do not offer access to any of the badly needed humanitarian supplies.

Indeed, admitted Egeland, “we do not have classical humanitarian corridors at all, in Aleppo, but we’re working with everybody, including Russia, to get those.”

Nonetheless he added that “the discussions have been very positive, in a sense that everybody now says that we are working towards the same goal of having two-ways humanitarian corridors, of having better protection of civilians, including medical installations.”

Egeland admitted that the number of civilians taking advantage of the escape corridors was “very limited so far.”

That was hardly surprising, according to medical workers, who told Fox News the corridors were targeted by regime snipers and were considered death traps to those who even came close to them.

Al Khateeb reported that a nine-year-old girl who strayed close to one of the corridor areas on Friday was shot in the stomach. She survived. Medical workers reported other recent anecdotal cases of civilians shot down in the corridors who were less lucky.

“You cannot trust anyone who is besieging you,” said Bakri Al Halabi, the head of an eastern Aleppo cultural Center, about claims that civilians could use the corridors safely. “It’s just a lie from the government in front of the United Nations.”