Islamic State fighters in Mosul are digging trenches and can be seen pouring oil in some of them to bolster their defensive positions in Iraq’s second largest city ahead of an expected U.S.-backed Iraqi ground assault on the city, the U.S. military said Friday.

ISIS is preparing for “hell on earth,” said Col. John Dorrian, a spokesman for the U.S.-led coalition, who added that the terror group is moving large walls to slow down the invasion.

Burning oil or tires produces a thick black smoke that can block the view from overhead, making it difficult for U.S. aircraft to visually identify potential targets.

Dorrian said ISIS has been burning oil fields near a U.S. base in Qayyarah, located 25 miles south of Mosul. He said ISIS was using the smoke to “mask movements.”

Fox News is told “if the stars align,” the Iraqi-led ground invasion could happen as soon as the next few weeks in mid-October.

Dorrian would not comment on reports that the U.S. military was planning on sending 500 more troops to Iraq, as first reported by The Wall Street Journal and confirmed by Fox News on Thursday. 

Located on the west bank of the Tigris River, about 40 miles south of Mosul, Qayyarah has since become an important staging ground for military and humanitarian efforts ahead of the Mosul operation since it was recaptured by Iraqi forces last month.

"It's the staging ground for military forces and it's where 350,000 of the 1 million people who are expected to flee (Mosul) will either find shelter or pass through," Lise Grande, the United Nations' humanitarian coordinator for Iraq, told The Associated Press.

There are slow-going Iraqi efforts to contain the fires, but nearly a month after the town was recaptured from the militants, smoke and toxic fumes continue to pollute the air in and around Qayyarah.

The Iraqi Oil Ministry spokesman, Assem Jihad, said Wednesday that ISIS militants set fire to 11 oil wells in Qayyarah to derail security forces and wreak havoc in the area as they fled. He said fires at nine of the wells have been extinguished, but two continue to burn powerfully.

The images of smoke and flames from the oil wells are reminiscent of the oil fires in Kuwait after the Iraqi military reportedly set fire to hundreds of wells when Saddam Hussein invaded the neighboring Persian Gulf nation in the early 1990s.

"In putting out the fires in Kuwait, the firefighters used water pipes and pumped the water from the Persian Gulf to spray at the base of the fires," said Kourosh Kian, an expert in petroleum drilling and reservoir engineering.

Kian, a system engineer at GE Aviation, said the simplest method to extinguish these types of fires is to inject water under high pressure at the base of the fire. Since Qayyarah is on the Tigris River, there would be no problem with the water supply, he said.

Qayyarah and Najmah, the two main fields in the area with reserves slightly over one billion barrels, came under the control of ISIS when it captured Iraq's Nineveh Province in June 2014.

While Iraqi forces now remain in control of the area, it is far from stable. At the Qayyarah West air base, where hundreds of U.S. troops are working to advise and assist their Iraqi counterparts, a small rocket that contained a mustard agent landed, Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told Congress on Thursday.

A U.S. official, who discussed details of the incident on Wednesday on condition of anonymity, said a small group of U.S. soldiers who inspected remnants of the rocket after it exploded found a black, oily substance on a fragment of metal. An initial test of the suspicious substance showed it contained residue of mustard agent, but a second test was negative.

Militants continue to dwell around the town to the west and along the eastern bank toward the town of al-Alam.

The Iraqi military, backed by coalition airstrikes and coalition advise-and-assist operations, looks to recapture more territory from the military group, which at one point in 2014 controlled about a third of Iraq and neighboring Syria. U.S.-led coalition forces have launched more than 460 airstrikes around Qayyarah since August 2014 and more than 1,800 around the city of Mosul itself.

But for aid workers in the country, the fires are an immediate primary concern as they prepare for a potential mass influx of displaced people as Mosul operations get underway.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.