London/Baghdad: The top boss of the Islamic State—who on Thursday denied the terror group was on the brink of losing control in the besieged northern Iraqi city of Mosul—has himself fled, Britain's Secretary of State for Foreign Affairs Boris Johnson said on Friday.

The Foreign Secretary told the House of Commons that Abu Bakr al-Baghdadi's call for militants not to retreat when faced with Iraqi forces was ‘cruelly ironic’ because intelligence suggested he had fled the city, the Daily Mail reported.

In an audio recording released on Thursday, al-Baghdadi said he was confident of victory in the de-facto capital Mosul and urged followers to continue fighting.

But Johnson, while responding to queries in the House of Commons, said: ‘It is a cruel irony that some of the intelligence we have, you may know, suggests that the gentleman in question has actually vacated the scene himself and is nonetheless using internet media to encourage people to take part in violence.’

He pledged that the IS would be repelled from the war-torn Iraqi city which has been controlled by the IS since June 2014, and told MPs: ‘The house can be sure that Daesh will be driven from Mosul, but this is the toughest task that Iraq's security forces have yet encountered.’

In his first statement since the major offensive began, al-Baghdadi implored jihadis to attack the ‘enemies of God’ in what is seen as an all or nothing battle for them.

‘Turn the nights of the unbelievers into days, to wreck havoc in their land and make their blood flow as rivers,’ he said in an audio release, the first since the major Iraqi offensive began.

The leader said he was ‘confident of victory’ and called on his fighters to resist as Iraqi forces were poised to enter the city, declared a ‘caliphate’ when it was overrun by the group two years ago.

In his latest message, which is undated but makes references to events that are at most a few weeks old, Al-Baghdadi calls for attacks against Saudi Arabia and Turkey.

Ankara has troops stationed at a base just outside Mosul and Turkish President Recep Tayyip Erdogan's escalating rhetoric has raised fears of a unilateral Turkish intervention in Iraq.

The US-led coalition estimates there are 3,000 to 5,000 IS fighters inside the city but the final outcome of the battle appears to be in little doubt.

Tens of thousands of Iraqi forces, backed by the US-led coalition and its warplanes, launched a massive offensive on Mosul on October 17.

More than 40,000 Iraqi and Kurd troops are leading the offensive, backed by air and ground support from a 60-nation US-led coalition, in what is expected to be a long and difficult assault.