°®Âþµº

Iraqi forces thrust into the Islamic State group-held city of Fallujah from three directions on Monday with an elite counter-terrorism unit leading the way.

It marks a new and perilous urban phase in the week-old operation to retake the extremist bastion, and could see casualties among government forces mount.

Led by the elite Counter-terrorism Service [CTS], Iraq's best trained and most seasoned fighting unit, the forces pushed in before dawn, commanders said.

"Iraqi forces entered Fallujah under air cover from the international coalition, the Iraqi air force and army aviation, and supported by artillery and tanks," said Lieutenant General Abdelwahab al-Saadi, the commander of the operation.

"CTS forces, the Anbar (provincial) police and the Iraqi army, at around 4am [local time], started moving into Fallujah from three directions," he said.

"There is resistance from [IS]," he added.

CTS spokesman Sabah al-Noman told AFP: "We started early this morning our operations to break into Fallujah."

Five things you need to know about Fallujah Rebel city Fallujah was once a small trading post on the Euphrates River, 50 kilometres (30 miles) west of Baghdad, but its aura in modern Iraq belies its relatively modest size. Sunni tribes were always powerful in Fallujah, whose reputation as a troublesome city predates the US-led invasion of 2003. In 1920, the murder there of a British officer was one of the sparks that ignited a nationwide revolt against the colonial power. The anti-British rebellion was the inspiration for the name of an armed group called the 1920 Revolution Brigades, which was founded in 2003 and still active in the Fallujah area in 2014 before it was swallowed up by IS. 'City of Mosques' Fallujah is an important religious hub for Iraq's Sunni minority. Its skyline bristles with hundreds of minarets that have earned it the nickname "City of Mosques". Built on a crossroads for routes from Saudi Arabia and Jordan, Fallujah was one of the first places in Iraq where hardline Wahhabi ideology took root. Now executed dictator Saddam Hussein jailed several radical preachers from Fallujah, although the city was generally not hostile to him and benefited from the policies of his Baath party regime that favoured Sunni Arabs. Blackwater Bridge On March 31, 2003, insurgents ambushed a convoy carrying four US contractors working for the private military company Blackwater. They were killed, their bodies dragged on the road and eventually hung from a bridge over the Euphrates. Photos of the mutilated bodies were beamed around the world, and remain among the most searing images of the US-led war in Iraq. The bridge became known as "Blackwater Bridge" and the incident jolted the world into an awareness of the violent reality that was going to prevail in Iraq, a year after the overthrow of Saddam. 'New Vietnam' Operation Phantom Fury was launched on November 7, 2004 and turned into the bloodiest battle US service personnel had seen since the Vietnam War. They went house to house in a bid to retake a city that had already become the capital of al-Qaeda in Iraq, a precursor of the Islamic State group that was founded by Abu Musab al-Zarqawi. The battle, in which 95 US troops were killed and more than 500 wounded, holds a special place in recent US military history. Varying estimates put the number of insurgents killed at between 1,000 and 1,500, and civilian casualties were believed to be in the hundreds. 'Head of the snake' Fallujah fell to anti-government fighters in early 2014 after security forces withdrew during unrest that began when they cleared a year-old anti-government protest camp near Ramadi, the capital of Anbar province, sparking fighting that later spread to Fallujah. The fall of the Fallujah, which later became a key IS stronghold, was the first time that anti-government forces had exercised such open control in an Iraqi city since the height of the violence that followed the US-led invasion. IS's broad offensive, in which second city Mosul was captured, did not happen until June 2014. Fallujah is seen by many Iraqis as the place where it all began and is sometimes nicknamed "the head of the snake".

The involvement of the elite CTS marks the start of a phase of urban combat in a city where in 2004 US forces fought some of their toughest battles since the Vietnam War.

The week-old operation had previously focused on retaking villages and rural areas around Fallujah, which lies just 50 kilometres (30 miles) west of Baghdad.

Only a few hundred families have managed to slip out of the Fallujah area ahead of the assault on the city, with an estimated 50,000 civilians still trapped inside, sparking fears the extremists could try to use them as human shields.

The only families who were able to flee so far lived in outlying areas, with the biggest wave of displaced reaching camps on Saturday night.

"Our resources in the camps are now very strained and with many more expected to flee we might not be able to provide enough drinking water for everyone," said Nasr Muflahi, the Norwegian Refugee Council's Iraq director.

"We expect bigger waves of displacement the fiercer the fighting gets."

Concern for civilians

On Monday, dozens of civilians, including women and children, were arrested by Iraqi forces as they tried to escape from villages in Fallujah. The army handed them over to local police and tribal militants to transfer them to camps for displaced people in the town of Amriyat al-Fallujah, 30 kilometres south of Fallujah.

Other civilians also trickled into the government-controlled town, starving and exhausted after walking through the countryside for hours at night, dodging IS surveillance.

"I just decided to risk everything. I was either going to save my children or die with my children," said Ahmad Sabih, 40, who reached the NRC-run camp early on Sunday.

Fallujah is one of just two major urban centres in Iraq still held by IS.

They also hold Mosul, the country's second city and de-facto extremist capital in Iraq, east of which Kurdish-led forces launched a fresh offensive on Sunday.

The extremists holed up in Fallujah are believed to number around 1,000.

It is not clear yet what resources IS is prepared to invest in the defence of Fallujah, which has been almost completely isolated for months, but the city looms large in modern jihadist mythology.

A centre of learning for Iraq's Sunni Muslim minority that has been known as "The City of Mosques", it was already a spark for a nationwide rebellion in 1920 against British colonial rule.

In November 2004, the US military suffered some of its worst losses in decades during an operation dubbed Phantom Fury which saw at least 95 members of the US forces killed in fierce combat against one of IS' previous incarnations.

Fallujah is expected to give Iraqi forces one of their toughest battles yet but IS has appeared weakened in recent months and has been losing territory consistently over the past year.

According to the government, the organisation that has sewn havoc across Iraq and Syria over the past two years now controls around 14 percent of the national territory, down from 40 percent in 2014.

However, as the "caliphate" it declared two years ago unravels, IS has been reverting to its old tactics of bombings against civilians and commando raids.

Agencies contributed to this report