The leader of Nigeria's Boko Haram, Abubakar Shekau, seen on May 12, 2014. (Associated Press)

The Islamic State appeared to endorse a pledge of loyalty from the Nigeria-based militant group Boko Haram on Thursday, signaling an alliance of interests between terrorist groups that share a reputation for brutality but are not known to have collaborated operationally.

In a speech distributed on social media, Abu Muhammad al-Adnani, a spokesman for the Islamic State, praised the recent declaration of allegiance by Boko Haram leader Abubakar Shekau.

The expressions of mutual admiration come as both organizations have committed a series of new atrocities even while facing expanded pressure from regional militaries seeking to oust the militant groups from separate territories they seized in Iraq and northern Nigeria.

The declared alliance is believed to be largely for propaganda purposes. U.S. intelligence officials said that they have seen no evidence of any operational ties between the groups, nor any willingness by Boko Haram to take orders from afar.

But both groups may see benefits to a declared alliance, even if it does not involve the transfer of any fighters or weapons.

Boko Haram leader Abubakar Shekau claims in a video, which could not be independently authenticated, that his fighters were behind the attack on a Nigerian town that killed scores of civilians. (Reuters)

The Islamic State “is trying to show that it’s this larger-than-life group” with affiliates on multiple continents expanding the footprint of its declared caliphate, a U.S. intelligence official said. The Islamic State also has drawn backing from groups in Libya and Afghanistan.

In his speech, Adnani warned that the group would respond to U.S.-led attacks on its territory with assaults on Paris and Rome and “blow up” icons including the Eiffel Tower and the White House, according to SITE Intelligence Group, which tracks the online postings of Islamist militant organizations.

For its part, Boko Haram may hope that its pledge will lead to financial backing from the Islamic State or a bump in recruits. “They wouldn’t be against accepting money from ISIL,” the official said, using another term for the Syria-based group. “But it’s unlikely Boko Haram would change the way it operates.”

The organization has built a reputation for brutality with a string of attacks including the kidnapping of hundreds of schoolgirls. But Boko Haram is under intense new pressure as troops from Niger, Chad and Cameroon have crossed into northern Nigeria to battle the insurgents. Nigeria also has reportedly called in mercenaries from South Africa and former Soviet republics to reinforce its troops.

A declared alliance between the groups may also complicate the debate in Washington over a new authorization sought by the Obama administration to continue to use U.S. military force against terrorist groups.

A White House proposal would allow President Obama to use all force he “determines to be necessary and appropriate against ISIL or associated persons or forces,” a construction that after this week’s developments could be more easily interpreted to allow attacks against Boko Haram.