Al Qaeda's wing in Yemen expressed support on Friday for Islamic State against what it described as a "crusade alliance", referring to US-led air strikes on the insurgents in Iraq and Syria, and called for attacks to damage the United States.
US and Arab forces have bombed Islamic State targets in Syria and Iraq since the militants declared a caliphate in the territory under their control.
Islamic State is an offshoot of Al-Qaeda but now challenges that group for influence among jihadists.
"We are behind our brothers against this international crusade and we join them in their enmity against this campaign," Al-Qaeda in the Arabian Peninsula (AQAP), the Yemeni branch of the movement, said in an online statement.
"We confirm the call to whoever is able to hurt the Americans to make an effort to hurt them militarily, economically and through the media. They are the leaders of this war and the base of this campaign."
AQAP reiterated its call on rival militant factions in Iraq and Syria to stop fighting each other and unite against the US-led alliance.
"We urge all the mujahedeen (Islamist fighters) to forget their differences and to stop the infighting among themselves," it said.
Islamic State has fought rival Islamist groups in Syria including Nusra Front, Al-Qaeda's official wing in Syria, that have sought to resist its expansion over the past year.
It has also waged a propaganda war with Al-Qaeda's central leadership in a thinly-veiled attempt to wrest leadership of global militancy from the group founded by Osama bin Laden, killed by US forces in 2011.
Islamic State fighters alarmed governments across the Middle East when they swept through northern Iraq, seizing cities, slaughtering prisoners, proclaiming a caliphate to rule over all Muslims and ordering non-Sunnis to convert or die.
The group, whose leader, Abu Bakr al-Baghdadi, calls himself "Caliph" or Muslim leader, fell out with Al-Qaeda in 2013 over its expansion into Syria, where his followers have carried out beheadings, crucifixions, and mass executions.
AQAP is one of the most active branches of Al-Qaeda, and militants have launched attacks from there against the West.