A letter found when al-Qaeda's chief in Iraq was killed said the group's leadership was based in Waziristan, Pakistan, the Washington Post reports.
Abu Musab al-Zarqawi brought the tactic of beheading to Iraq
The December 2005 missive is said to be the first to emerge from what the US military calls a "treasure trove" after Abu Musab al-Zarqawi's death in June.
It was reportedly sent by a member of Osama Bin Laden's high command, who said he wrote from Waziristan.
Bin Laden is suspected of hiding along the Afghan-Pakistani border.
If accurate, the letter would confirm the location of the al-Qaeda leadership at the time it was written, the newspaper said.
'Wish to advise'
The missive was uncovered from Iraqi safe houses at the time of Zarqawi's death in a US air strike, according to the Washington Post.
A 15-page English translation of the Arabic document was released last week by a US military counter-terrorism centre, the paper reported.
The author said he was writing from al-Qaeda headquarters in the restive border region, where Taleban and al-Qaeda fugitives have been active.
The paper said the letter was signed by "Atiyah," whom counter-terrorism officials believe is Atiyah Abd al Rahman, a 37-year-old Libyan who joined Bin Laden during the 1980s.
"I am with them," the letter says.
"And they have some comments about some of your circumstances."
The letter described the difficulty of direct communications between Waziristan and Iraq, and suggested it was easier for Zarqawi to send a representative to Pakistan than the other way around, the Washington Post reported.
It also warned Zarqawi that he risked removal as the leader in Iraq if he continued to alienate Sunni leaders and rival insurgent groups, the paper said.
The "brothers wish that they had a way to talk to you and advise you, and to guide and instruct you; however, they too are occupied with vicious enemies here," Atiyah reportedly wrote.
"They are also weak. And we ask God that He strengthen them and mend their fractures."
Counter-terrorism officials reportedly deemed the document authentic.
The report comes a day after the release of a video purportedly showing the ringleader of the 9/11 attacks.
British newspaper the Sunday Times posted the video, which has no sound, on its website.
Dated January 2000, it shows ringleader Mohammed Atta and fellow hijacker Ziad Jarrah talking and laughing, and speaking to the camera.
The Sunday Times said the video was made in Afghanistan for release after their deaths.