The Islamic State has accepted a pledge of allegiance to the group made by the Nigerian jihadists Boko Haram, according to an audio recording purportedly from its spokesman.
"We announce to you to 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," IS spokesman Mohammed al-Adnani said in the message, using the name in Arabic of the Nigerian group.
Itself a radical Sunni Muslim movement, IS has seized large swathes of Iraq and Syria and declared an Islamic caliphate there, and has also drawn expressions of allegiance from jihadists in Egypt and Libya.
On Saturday, an audiotape attributed to Boko Haram leader Abubakar Shekau said "we announce our allegiance to the Caliph of the Muslims, Ibrahim ibn Awad ibn Ibrahim al-Husseini al-Qurashi," referring to IS leader Abu Bakr al-Baghdadi.
Shekau has previously mentioned Baghdadi in video messages yet stopped short of pledging formal allegiance.
But there have been increasing signs that the Nigerian militants, whose six-year insurgency has claimed more than 13,000 lives and left 1.5 million people homeless, have been seeking a closer tie-up.
The IS spokesman urged Muslims to join militants in West Africa and insisted that the caliphate was growing.
"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.
He insisted that the jihadist group is "sure of its victory" regardless of the challenges it is facing.
Meanwhile, an intelligence agent working for one of the states in the US-led coalition fighting IS has been captured for helping three British teenage girls cross into Syria to join the militants.
Turkey’s foreign minister Mevlut Cavusoglu said in an interview: "Do you know who helped those girls? He was captured. He was someone working for the intelligence (service) of a country in the coalition"
Mr Cavusoglu said in an interview that the agent was neither a national of an EU state, nor the US, without specifying further.
"It's not an EU member, it's also not the United States. He is working for the intelligence of a country within the coalition," he said.
He also did not say where or how the alleged spy had been captured.
Mr Cavusoglu said he had informed his British counterpart Philip Hammond of the development.
"He told me 'just as usual'," said Mr Cavusoglu, without explaining further.
Kadiza Sultana, 16, and 15-year-olds Shamima Begum and Amira Abase, crossed into Syria after boarding a flight from London to Istanbul on 17 February.
They took a bus from Istanbul to the southeastern Turkish city of Sanliurfa close to the Syrian border from where they are believed to have crossed the frontier.
Mr Cavusoglu's comments come amid growing irritation among Turkish officials over repeated criticism from the West that Ankara is not doing enough to stop jihadists and their sympathisers crossing from Turkey into Syria.
Turkey had last month accused Britain of a "reprehensible" delay in informing the Turkish authorities over the departure to its territory of the three teenage girls.
Turkish officials say Ankara has stepped up efforts to stop the flow of jihadists but needs to be given intelligence by Western partners in advance of their arrival in order to stop them.
Along with the US and EU states, Arabian peninsula nations including Saudi Arabia and Qatar have been involved in the coalition against IS.
