The Iraqi army announced Thursday that it has launched an operation to retake the city of Mosul, the Islamic State jihadist group's main hub in Iraq.

Across the border, the Syrian army and allied militia were battling to recapture the ancient city of Palmyra in a final assault.

Here is a recap of key cities and towns in Iraq and Syria seized by IS or recaptured from them:

- IRAQ -

MOSUL: The capital of Nineveh province, it lies 350 kilometres (220 miles) north of Baghdad. IS captured it on June 10, 2014.

It is by far the largest urban centre in the "caliphate" which the group proclaimed nearly two years ago. Two million people lived there before IS arrived, but hundreds of thousands have since left.

The Iraqi army said it launched the initial phase of an operation to recapture Mosul, making small territorial gains around 60 kilometres (35 miles) south of the city itself.

RAMADI: The Sunni Arab city 100 kilometres (60 miles) west of Baghdad is the capital of Anbar, the country's largest province, which stretches from the borders with Syria, Jordan and Saudi Arabia to just west of the capital.

IS seized Ramadi in mid-May, 2015 in an assault involving dozens of suicide bombers driving explosives-rigged vehicles. Iraqi forces launched an operation to retake the city late last year and declared full control over the area last month.

TIKRIT: Hometown of late Iraqi dictator Saddam Hussein located 160 kilometres (100 miles) north of Baghdad. It was recaptured in April by Iraqi troops, police and Shiite-dominated paramilitaries. The operation, at that time the largest by Iraqi forces against IS, was helped by the fact that much of Tikrit's civilian population had fled the city.

SINJAR: Iraqi Kurdish forces backed by US-led coalition air strikes recaptured Sinjar, 400 kilometres (250 miles) northwest of Baghdad, from IS on November 13. That cut a key supply line to areas held by the jihadists in Iraq and Syria. IS captured Sinjar in August 2014 and carried out a brutal campaign against its Yazidi minority that included massacres, enslavement and rape.

BAIJI: Iraqi forces recaptured the Sunni Arab town of Baiji, 200 kilometres (120 miles) north of Baghdad and a nearby refinery, Iraq's largest, in mid-October.

Baiji was the scene of some of the longest-running battles with IS in Iraq. It lies at a major crossroads and its recapture was seen as key to preparing the ground for offensives in Anbar and Mosul.

- SYRIA -

RAQA: Home to 300,000 people, Raqa is IS's de facto Syrian capital. It is a major target of air strikes by the US-led coalition, as well as Syrian and Russian aerial bombardment.

PALMYRA: This ancient Syrian city, known as "the Pearl of the Desert", is located some 200 kilometres (130 miles) east of Damascus. It was taken by IS in May. The group has since razed and looted UNESCO-listed archeological gems and beheaded the city's former antiquities chief.

KOBANE: A Kurdish city in northern Syria on the Turkish border. It became a symbol of the fight against IS and its liberation marked the group's first serious setback since it began to advance in the country in 2013. IS fighters were driven out of Kobane on January 26 after more than four months of fierce fighting with Kurdish forces backed by US-led strikes.

The city, known in Arabic as Ain al-Arab, is the capital of one of three semi-autonomous "cantons" established by Kurds after the Syrian war erupted.

TAL ABYAD: Another city on the Turkish border, it was captured by Kurds on June 16, dealing IS one of its most serious defeats to date. Tal Abyad had 130,000 inhabitants when the Syrian conflict began in 2011, and lies on a key supply route between Turkey and Raqa. IS fighters and arms regularly passed through the city before its recapture.

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