Besieged Islamic State militants in the Iraqi city of Ramadi destroyed a lock on the Euphrates River that served as a bridge as government forces on Thursday sought to cement their gains around the militant-held city west of Baghdad.

Since Iraq’s military launched its push on Ramadi earlier this month, the militants have destroyed all other bridges leading into the city, both on the Euphrates and its tributary, the Warar River.

Iraqi Maj Gen. Ismail al-Mahlawi, the head of military operations in the western Anbar province, said the lock destroyed Thursday was the last remaining bridge from the city center to the northwest.

“Daesh forces trying to stop our progress bombed the last bridge which connects the city center,” he said, referring to IS by its Arabic language acronym.

The lock’s destruction leaves some 300 IS fighters trapped in the center of the city, he added.

Col. Steven Warren, the spokesman for the US-led coalition in Baghdad, said the destruction of the bridge may prove to be a tactical mistake for IS.

“What they’ve also done now is they’ve really cut themselves off,” he said. “So the fighters left on the north side of the river can’t retreat and the fighters on the south side of the river can’t send reinforcements.”

Muhannad Haimour, the spokesman for the Anbar governor’s office, said he received reports from residents still inside Ramadi that IS was also destroying buildings and radio towers.

“We’ve seen this before; they tend to blow up not just bridges, but a lot of infrastructure inside the city,” Haimour said.

Haimour added that according to reports he received, about two months ago, IS fighters began moving their families out of Ramadi and toward the town of Hit northwest of Ramadi. That, he said, is when he believes the tide began to turn against the IS group in the Anbar provincial capital.

A key factor that changed the sluggish pace of the battle for Ramadi, Haimour said, was a decision by the central government in Baghdad to arm Sunni tribal fighters from the Ramadi area to fight against IS.

“They didn’t feel like they had enough support from the coalition and the central government, but all of that changed a few months ago,” Haimour said. Now there are 8,500 members from Anbar mobilised, trained, armed and receiving salaries.

While Iraqi forces were consolidating their gains, Warren, the coalition spokesman, said they also successfully repelled a number of IS counter-attacks Thursday with “significant” coalition air support.

In a statement, the US-led coalition said six airstrikes targeted IS units, boats and fighting positions near Ramadi on Wednesday. Over the past week, coalition planes have launched 36 strikes near Ramadi.

But as the operation to retake the provincial capital progresses, Ramadi’s sizeable civilian population – estimated to be between 4,000 and 10,000 – remains mostly trapped inside the city. Iraqi officials say they believe civilians will be able to flee the city, but coalition officials report that so far they have only witnessed small groups do so.

IS captured Ramadi in May and though the government immediately announced a counter-offensive, progress in retaking the Sunni heartland of Anbar has been slow. Iraqi forces, backed by US-led airstrikes, pushed into Ramadi earlier this week, capturing a military complex north of the city and a neighbourhood on its outskirts. (AP)

•Photo by Getty Images shows Iraq's counter-terrorism forces in the al-Tameem district of Ramadi.