"It is not possible that many of the people of the south (Shia) violate, alongside America and its allies, [the Sunni cities of] Ramadi, Falluja, Mosul ... that their areas would be safe from retaliation and harm," said the speaker on the audiotape posted on the net.
In what would be his second message in two days if confirmed, bin Laden said the Islamic community was depending on Islamist fighters in Iraq as well as Somalia.
"Your Muslim nation is looking for you and praying for your victory. You are their hope after God. You are God's trusted soldiers who will liberate the ummah (the Muslim nation) from the serfdom of the crusaders in our countries."
The al-Qaeda leader also said his group reserved the right to fight the United States and warned Washington and the international community against sending forces to Somalia.
"We will fight its (US) soldiers on the land of Somalia ... and we reserve the right to punish it on its land and anywhere possible.
Warning against foreign troops
"We warn all of the countries in the world not to respond to America by sending international troops to Somalia."
He called on all Somalis to support the Islamic Courts Union, which he said were building an Islamic state in the Horn of Africa.
"You have no other means for salvation unless you commit to Islam, put your hands in the hands of the Islamic Courts to build an Islamic state in Somalia," he said.
"We have been informed that our mujahideen brothers at (Iraq's) al-Qaeda have chosen Abu Hamza al-Muhajir as their emir ... I pray God that he will be the best successor of the best predecessor"
Message attributed to Osama bin Laden
Bin Laden also lashed out at the president of Somalia's secular interim government, Abdullahi Yusuf, calling him a "traitor" and a "renegade".
At just over 19 minutes in length, the audio recording was accompanied by a photo of bin Laden above the logo and name al-Sahab, al-Qaeda's media production wing.
On Friday, bin Laden paid tribute to slain leader of al-Qaeda in Iraq, Abu Musab al-Zarqawi, and denied that his organisation was involved in sectarian strife in Iraq.
Bin Laden also endorsed Abu Hamza al-Muhajir, also believed to have the name of Abu Ayyub al-Masri, as the new leader of al-Qaeda in Iraq after the killing of Abu Musab al-Zarqawi in a US air strike on June 7.
"We have been informed that our mujahideen brothers at (Iraq's) al-Qaeda have chosen Abu Hamza al-Muhajir as their emir ... I pray God that he will be the best successor of the best predecessor," he said.
Bin Laden ordered al-Muhajir to focus the fight "on the Americans and their allies" and advised him not to break away from the Mujahideen Shura Council, an umbrella of Sunni groups led by al-Qaeda. "Dispute is evil as a whole and (being part of) a group is mercy," he said.
He urged Muslims to send men and funds to support fighters in Iraq. "Muslims should rescue their brothers in Mespotamia with money and men to deter the aggression of the crusaders and apostates," he said.