The besieged civilian population of Fallujah is confronting a “human catastrophe” as a US-backed offensive to retake the Iraqi city from the Islamic State of Iraq and Syria (ISIS) intensifies, a human rights group warned Tuesday.

While forces backing the Iraqi government, including troops of the elite Counter Terrorism Service, Iraqi Army soldiers, police and Shia militiamen of the Popular Mobilization Units, have moved to the outskirts of the city, stiff ISIS resistance Tuesday prevented them from advancing into its center. At least 50,000 civilians are believed to be trapped in Fallujah.

“A human catastrophe is unfolding in Fallujah. Families are caught in the crossfire with no safe way out,” warned Jan Egeland, Secretary General of the Norwegian Refugee Council (NRC), which has provided aid to civilians in the area.

“The stories coming out of Fallujah are horrifying,” said Nasr Muflahi, NRC’s Country Director in Iraq. “A lack of food, medicine, safe drinking water and electricity are pushing families to the brink of desperation. People who managed to flee have told us of extreme hunger and starvation. We haven’t been able to see this for ourselves or assist people inside the town, and we are extremely concerned about the full extent of the terrors unfolding there.”

The United Nations’ humanitarian aid director in Iraq, Lise Grande, further warned that the city could be only “days away from a cholera outbreak,” because of the lack of clean drinking water.

Fallujah has been under siege for close to a year, with roads that bring in vital supplies cut by the Iraqi Army and Shia militias. Now, its residents are facing intensifying bombardment from US and allied warplanes, Apache attack helicopters and Iraqi artillery.

Fallujah was the first major Iraqi city to fall to ISIS at the beginning of 2014, six months before the Islamist militia overran Mosul, Iraq’s second largest population center, along with roughly a third of Iraqi territory.

The early victory in Fallujah was made possible by a revolt on the part of the city’s Sunni population against the Shia-dominated central government in Baghdad, which was widely reviled for carrying out sectarian repression against Sunnis.

While there have been widespread reports of ISIS exploiting the city’s civilians as “human shields”—a charge frequently made by the US military to provide an alibi for carnage inflicted by American air strikes—it is also reported that the bulk of the ISIS fighters are city residents.

This marks the third time in a little over a decade that Fallujah has been subjected to an all-out military siege. Twice in 2004, the US Marines, backed by heavy aerial bombardments, stormed the city, killing thousands and reducing the bulk of Fallujah’s homes and infrastructure to rubble. A center of resistance to the 2003 US invasion and subsequent occupation of Iraq, the city was subjected to merciless collective punishment.

Fallujah’s fate this time around has been presaged by that of previous Iraqi Sunni towns retaken by forces loyal to the Baghdad government. These include Ramadi, where at least 70 percent of the buildings were destroyed by bombardment and the population of 400,000 has been driven out, and Tikrit, where Shia militias carried out bloody reprisals against the population for the atrocities committed by ISIS, which included the massacre of up to 1,700 predominantly Shia military cadets.

Sunnis view this US-backed campaign as an exercise in ethnic cleansing designed to expel them from Iraq. The massive destruction unleashed on these cities, however, has also been seen in the predominantly Kurdish town of Kobane, which was largely razed to the ground, as well as the Yazidi town of Sinjar. It is a function of the type of warfare employed by the Pentagon, in which proxy ground forces, often with the participation of US special operations troops, rely on heavy air support to defeat ISIS.

While the US military is supporting the offensive against Fallujah with intense air strikes, the Pentagon had reportedly opposed the move against the city, seeing it as a distraction from the buildup for an attack on Mosul, Iraq’s second city, which had a population of some 2 million before falling to ISIS in June 2014.

Washington is also uneasy about the prominent role being played by Iranian advisers on the ground as well as the Shia militias, which provide much of the manpower for the siege. The US views Iran as its major regional rival for hegemony over the Middle East in general, and Iraq in particular.

For Iraq’s Prime Minister Haidar al-Abadi, the siege of Fallujah is seen as a political imperative for his government, which has faced mounting popular opposition from within Baghdad’s impoverished Shia majority. Crowds have twice stormed the heavily fortified Green Zone, the seat of the central government, to protest rampant corruption and the failure to ensure essential services.

Also fueling the growing popular anger is a series of terrorist attacks centered in poorer Shia neighborhoods. The government has charged that Fallujah, less than 40 miles west of the capital, is the center from which these attacks are planned and executed.

Unfolding parallel to the siege of Fallujah is a separate anti-ISIS operation to the north also backed by extensive US-led air strikes. This operation, directed at preparing an offensive against Mosul, is being conducted largely by Kurdish Peshmerga fighters and allied militias, with the participation of US special forces, who are increasingly engaged in combat.

Similarly, Kurdish paramilitaries of the YPG (Kurdish People’s Protection Units) are providing the main ground forces for a campaign across the border in Syria aimed at re-taking the city of Raqqa, the capital of ISIS’s self-styled caliphate.

While the Kurdish forces, supported by US special operations “advisers” and US warplanes, are advancing from the northwest, the Syrian military, backed by Russian airpower, is advancing from the southwest.

This race for Raqqa reflects the underlying conflict between Washington and Moscow over Syria, with the US backing a war for regime change against the government of President Bashar al-Assad, and Russia working in alliance with the government.

In both Iraq and Syria, Washington’s reliance upon Kurdish forces has antagonized its NATO ally, Turkey, which has demanded that the US brand Syria’s Kurdish militia as “terrorist” because of its ties with the Turkish Kurdish PKK (Kurdistan Workers Party), against which the Turkish military is waging a simmering civil war.

Not only has the Obama administration resisted Ankara’s pressure on this score, the chief of the US Central Command paid a visit to the Kurdish units in northern Syria last month. Subsequently, photographs of US special forces wearing YPG patches on their uniforms provoked fresh outraged protests from the Turkish government.

At the same time, the prospect of Kurdish forces “liberating” either Raqqa in Syria or Mosul in Iraq, both predominantly Sunni cities, has sparked new fears of ethnic cleansing and partition.

The advance of the disparate anti-ISIS offensives has served to underscore the catastrophic destruction inflicted upon the region by US imperialism, which deliberately incited sectarian divisions, first as part of its divide-and-rule strategy in Iraq, and then to promote a sectarian-based war for regime change in Syria.

At the same time, the prospect of defeating ISIS only exposes even more clearly the mutually opposed interests of the various outside powers that claim to be united in their opposition to the Islamist militia, raising the specter of the present conflict spawning regional and even world war.