Syrian rebels have reportedly launched a major offensive toward the home province of President Bashar al-Assad, as the UN relief coordinator presented yet another dismal report about humanitarian conditions in the war-torn country.

The Army of Conquest, an alliance of Sunni Islamist militias that includes the al Qaeda affiliate Jabhat al-Nusra, announced the offensive via Twitter on Sunday. The group, which has already taken over much of Idlib province, vowed to "rain down a barrage of rockets and burning lava" on the native region of Assad's family, Qardaha, "so that Alawite villages can taste the evil that the Assad regime and Hezbollah's hands have done," according to a translation of the statement by a Lebanese news website, NOW.

The Syrian Observatory for Human Rights (SOHR) reported Tuesday that the militias have taken territory in the strategically important Sahl al-Ghab plain, an area just east of a mountain range that protects Qardaha and the Mediterranean coast from rebel-controlled areas. Cities and villages along the coast are home to many Syrian Christians and members of Assad's own sect, the Alawites, a Shiite group regarded as heretical by fundamentalist Sunnis. These groups form the core of support for Assad's regime, with many fearing persecution at the hands of the hard-line Sunni groups that now lead the resistance.

According to a Syrian military source who spoke with Reuters, the offensive is both large and widespread. The Army of Conquest claimed on Twitter that they now hold several villages on the plain and around the city of Jisr al-Shughour to the north, as well as a nearby power station. The SOHR said that clashes are continuing, and that two suicide bombers from Jabhat al-Nusra have detonated themselves in the area.

The setbacks for the government come after Assad admitted that government forces suffer from "a shortfall in human capacity," and would have to be selective about which areas they try to hold. However, the government does not appear ready to relinquish the Sahl al-Ghab plain and is responding with air raids, barrel bombs, and shelling, according to the SOHR.

The rebels say that their drive toward the Alawite heartland is vengeance for the regime's assault on the town of Zabadani, which sits between the capital of Damascus and the Lebanese border. Over the past three weeks, Syrian government forces have pummelled the town with barrel bombs while their Hezbollah allies moved in with ground troops, leading to an "unprecedented level of destruction and deaths among civilians," according to UN emergency relief coordinator Stephen O'Brien.

O'Brien addressed the UN Security Council on Tuesday to highlight the continued plight of civilians across Syria, now in its fifth year of civil war. The war's death toll has now topped 220,000, he said, with more than 1 million people driven from their homes in 2015 alone. That brings the total number of internally displaced people to well over 8 million, according to O'Brien, with an additional 4 million refugees in neighboring countries.

"Syria today," O'Brien said, "is the most acute, unrelenting and shameful blot on the world's humanitarian conscience."

He warned that relief workers are still unable to access large parts of the country, where approximately 80 percent of the population lives in poverty and most grapple with food insecurity and a lack of access to basic services, according to his report. He said that the regime continues to bomb medical facilities, while the rebels have cut off the water supply in much of the city of Aleppo, exacerbating the spread of disease.

As he wrapped up his report, O'Brien called on the Security Council to "consider its options through their eyes, the eyes of the beleaguered, now long-suffering Syrian people." International action has so far been blocked by dissension between the permanent members of the 15-nation council, which alone can impose binding sanctions or authorize military action against a sovereign country.

"A political solution is more urgent than ever to end this futile, hopeless cycle of brutality and violence," O'Brien said. "We must show the people of Syria that the world has not forgotten them or the plight of their country."

Follow Arthur White on Twitter: @jjjarthur