Video: Iraqi forces close in on Ramadi

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

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 Major-General Ismail al-Mahlawi, the head of military operations in western Anbar province, said the lock destroyed on Wednesday was the last remaining bridge from the city centre to the northwest.

Residents still inside Ramadi said IS was destroying buildings and radio towers

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

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

Colonel 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 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, 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 towards the town of Hit to the northwest.

That, he said, is when he believed the tide began to turn against 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 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 on 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 soon be able to flee the city, but coalition officials report that so far they have only witnessed small groups doing 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.