Iraqi forces push deeper into western Mosul as civilians flee

* Troops entering more built-up areas near airport

* Islamic State resisting with bombs and snipers

* Residents report clashes between local, foreign fighters

* Losing Mosul would be hammer blow to IS caliphate (Adds civilian accounts, journalist killed)

By Isabel Coles

MOSUL, Iraq, Feb 25 (Reuters) - U.S.-backed Iraqi forces pushed deeper into western Mosul on Saturday, advancing in several populated southern districts after punching through the defences of Islamic State's last major urban stronghold in Iraq a day earlier.

About 1,000 civilians walked across the frontlines, the largest movement since the new offensive launched last week to deal the ultra-hardline Sunni Muslim group a decisive blow.

In the capital Baghdad, Saudi Foreign Minister Adel Al-Jubeir met Iraqi Prime Minister Haider al-Abadi in the first such visit in more than a decade between Sunni Muslim-ruled Saudi Arabia and Shi'ite-led Iraq.

The new push in Mosul comes after government forces finished clearing Islamic State from the east of the city last month, confining the insurgents to the western sector across the Tigris river.

Commanders expect the battle there to be more difficult, in part because tanks and armoured vehicles cannot pass through the narrow alleyways that crisscross ancient districts.

But Iraqi forces have so far made quick advances on multiple fronts, capturing the northern city's airport on Thursday, which they plan to use as a support zone, and breaching a three-metre high berm and trench set up by Islamic State.

The advancing forces are less than three kilometres (two miles) from the mosque where Islamic State leader Abu Bakr al-Baghdadi declared a caliphate spanning Iraq and Syria in 2014, sparking an international military campaign to defeat the group.

Losing Mosul would likely deal a hammer blow to the militants' dream of statehood, but they still control territory in Syria and patches of northern and western Iraq from where they could fight a guerrilla-style insurgency in Iraq, and plot attacks on the West.

Federal police and an elite Interior Ministry unit known as Rapid Response have recaptured Hawi al-Josaq along the river and begun clearing the Tayyaran district north of the airport, said Brigadier General Hisham Abdul Kadhim.

Islamic State resisted with snipers and roadside bombs, he said. A Reuters correspondent saw two militants' corpses outside a mosque in Josaq.

FOREIGN FIGHTERS

Counter-terrorism forces also pushed on two fronts towards Wadi Hajr and Mamoun districts, said Lieutenant General Abdelwahab al-Saadi, a senior commander.