U.S.-backed Iraqi forces launched a large-scale military operation on Sunday to retake the western half of Mosul and dislodge Islamic State militants. It is the latest phase in a 4-month-old offensive to retake Iraq’s second largest city.

Prime Minister Haider al-Abadi announced the start of the operation on state TV, saying government forces were moving to "liberate the people of Mosul from Daesh oppression and terrorism forever," using the Arabic acronym for ISIS. He called on security forces to deal with civilians properly and to respect human rights.

Iraqi forces declared eastern Mosul “fully liberated” last month, however ISIS militants continued to launch attacks there. Hours after the latest operation was announced, suicide bombers struck troops and pro-government Sunni militiamen in eastern Mosul.

"ISIS's cruelty, brutality and reach show they are not just a threat in Iraq and Syria, but to the region and the entire world," said Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force Operation Inherent Resolve, in a statement.

Plumes of smoke were seen rising into the sky early Sunday morning as U.S.-led coalition jets struck militant positions southwest of Mosul and militarized Iraqi police fired artillery toward the city. Heavily armed police units were getting ready to move north with their armored vehicles from a base just southwest of the city.

"This is zero hour and we are going to end this war, God willing," said Mahmoud Mansour, a police officer, as he prepared to move out.

The battle for western Mosul promises to be the most daunting yet, as the half of the city west of the Tigris River has older, narrower streets and is still home to hundreds of thousands of civilians, who have been told to shelter in place.

"Mosul would be a tough fight for any army in the world, and the Iraqi forces have risen to the challenge," said Townsend. "They have taken the fight to the enemy and sacrificed their blood for the people of Iraq and the rest of the world."

The immediate objective was to take the villages on the southern outskirts of Mosul airport, a police spokesman told The Associated Press. The officer spoke on condition of anonymity because he was not authorized to brief reporters.

Police units quickly moved into the village of Athba, about 3 miles (5 kilometers) southwest of the airport, encountering only light resistance, according to an AP reporter traveling with them. Separately, the army's 9th Division moved into the village of Bakhira, also southwest of the city, the Iraqi Ministry of Defense said.

The United Nations meanwhile warned that hundreds of thousands of civilians trapped inside their houses "are at extreme risk," with dwindling fuel and food supplies and scare drinking water and electricity.

"The situation is distressing. People, right now, are in trouble," Lise Grande, the U.N. humanitarian coordinator for Iraq, said in a statement. "We are hearing reports of parents struggling to feed their children and to heat their homes," Grande said.

Citing witnesses in western Mosul, the U.N. said nearly half of all food shops were closed and bakeries had shut down due to a lack of fuel and an inability to purchase costly flour. Prices of kerosene and cooking gas have skyrocketed, and many of the most destitute families are burning wood, furniture, plastic or garbage for cooking and heating.

"Three out of five people now depend on untreated water from wells for cooking and drinking as water systems and treatment plants have been damaged by fighting or run out of chlorine," said Peter Hawkins, of the U.N. agency for children.

The humanitarian agencies were gearing up to aid 250,000 to 400,000 civilians who may flee due to fighting, the statement said. The U.N. estimates that about 750,000 civilians may be left in western Mosul.

Iraqi forces spent three months driving ISIS from eastern Mosul, but the militants appear to have left sleeper cells to carry out attacks behind the front lines.

Brig. Gen. Yahya Rasool, an Iraqi military spokesman, said a suicide bomber set off struck a patrol of Sunni tribal fighters in Zihoor neighborhood, while another targeted Iraqi troops in Nabi Younis.

Rasool declined to provide casualty figures. Two policemen said one Sunni fighter was killed and nine wounded in the first attack, while the second attack wounded five soldiers. They spoke on condition of anonymity as they were not authorized to release information.

Iraqi special operations forces, regular army and federal police units are taking part in the offensive along with government-approved paramilitary forces, mainly consisting of Shiite militias, which are operating on the city's outskirts.

Mosul fell to ISIS in the summer of 2014, along with large swaths of northern and western Iraq. It is the extremist group's last major urban bastion in Iraq.

The Associated Press contributed to this report.