Fugitive al-Qaeda leader Osama Bin Laden has apparently warned Iraqi Shia Muslims not to attack Sunni Muslims, saying they could face retaliation.
Five messages from Bin Laden have been reported this year
In an internet audio message to militants in Somalia and Iraq, the speaker calls Shia leaders traitors.
The voice warns the world community against sending troops to Somalia, where Islamists have made major gains.
If confirmed, the message is the second in two days released by Bin Laden, and his fifth audio message this year.
However, no new video images of the al-Qaeda leader have appeared since October 2004.
Washington said it was trying to determine the authenticity of the message.
"If authentic, the tape demonstrates yet again that Bin Laden and al-Qaeda continue to use the media to justify their dark vision and war against humanity," a White House official said.
The 19-minute recording tells fighters in Iraq that they are "God's trusted soldiers who will liberate [Muslims] from the serfdom of the crusaders".
The message says Sunni Muslims in Iraq are being annihilated and warns that Shia areas will not be safe from retaliation if attacks on Sunni cities continue.
Correspondents say the message seems to be timed to undermine Prime Minister Nouri Maliki, who is on a tour seeking support from Sunni Arab countries for his national reconciliation plan aimed at stemming sectarian tensions and violence.
Since Saddam Hussein was overthrown in 2003, the minority Sunni elite have been replaced by Shia leaders who could claim legitimately that they represented the majority of Iraq's population.
Insurgents have attacked Shia Islam's most important shrines at Karbala, Najaf and Samarra and killed many Shia politicians, clerics, soldiers, police and civilians.
However, Shia militia groups are also blamed for many sectarian attacks against Sunnis in Iraq.
In the message the speaker also endorses the new leader of al-Qaeda in Iraq Abu Hamza al-Muhajir, who was appointed to replace Abu Musab al-Zarqawi, killed three weeks ago.
Last week a video was broadcast purportedly showing the deputy leader of al-Qaeda, Ayman al-Zawahri, in which he paid tribute to Zarqawi and said his death would be avenged.
The speaker on the recording also calls on all Somalis to back the Council of Islamic Courts movement in its bid to build an Islamic state in Somalia.
The Islamists are the first group to control Mogadishu in 15 years
Since Islamic Courts militias banished secular warlords last month from the Somali capital, Mogadishu, Somalia is suddenly of much greater interest to al-Qaeda and Bin Laden, BBC Africa analyst David Bamford notes.
The Council of Islamic Courts is led by Sheikh Hassan Dahir Aweys who has been on the US list of people "linked to terrorism" since shortly after the 11 September 2001 attacks.
However, he denies any links to terrorism.
Bin Laden's apparent intervention may not be welcome news to Somalia's new lslamist leaders, our correspondent adds.
They have, he says, been at pains in recent days to convince Washington and regional African governments that they pose no threat, despite their Islamic leanings, and want to bring stability to Somalia.