On Thursday the extremists released the half-hour recording, purported to be from "Islamic State" (IS) spokesman Abu Mohammad al-Adnani.

Adnani pledged IS would "blow up the White House, Big Ben and the Eiffel Tower" before taking over other major world cities, such as Rome and Jerusalem.

"We will make your lives black and dark," he said.

He also predicted the group would expand further into the Middle East, into countries such as Afghanistan, Pakistan and the United Arab Emirates.

IS has taken control of major parts Iraq and Syria.

Adnani denied that the extremist group had suffered setbacks, saying it was actually growing in strength.

"Our caliphate is resisting and it is advancing in the right direction. We are fighting the Crusaders and the rafidah (Shiites) and day by day the Islamic State is becoming strong," he said.

The tape was released to coincide with the announcement IS had "accepted the allegiance" of Nigerian militants Boko Haram.

A message on Saturday attributed to Boko Haram leader Abubakar Shekau proclaimed the group's loyalty to IS.

Although he had mentioned IS leader Abu Bakr al-Baghdadi in previous videos, this was the first official declaration of formal allegiance.

Adnani's reply in Thursday's tape said that they welcomed Boko Haram's pledge, using the group's Arabic name, which means they can spread their control to parts of Africa.

"We announce to you the good news of the expansion of the caliphate to West Africa because the caliph (…) has accepted the allegiance of our brothers of the Sunni group for preaching and the jihad."

The message comes as the United States is drawing up a draft resolution to put together a regional force to fight Boko Haram.

Iraqi forces are planning a coordinated attack on the city of Tikrit on Friday, after entering the IS-held city on Wednesday.

Adnani insisted that "the victories proclaimed by coalition are only illusions," and only "inches of land" had actually been recaptured.

an/msh (dpa, AFP)