The Nation quoted a report published in The Washington Post as saying that the author of the December 11, 2005 letter, who claimed he was writing from al-Qaeda headquarters in the Waziristan region, was a member of Osama bin Laden's high command who signed himself ''Atiyah''.
The military's Combating Terrorism Centre at West Point, which last week released a 15-page English translation of the Arabic document made public in Iraq, said his real identity was ''unknown,'' according to The Washington Post.
Zarqawi was killed in June when US warplanes bombed his hideout in a village north of Baghdad.
Pakistan has maintained that it believes Osama bin Laden and his top lieutenants are probably on the Afghan side of the border, a point repeatedly made by President Gen Pervez Musharraf during his recent visit to the United States.
US counter-terrorism officials believe that ''Atiyah'' is Atiyah Abd al Rahman, a 37-year-old Libyan who joined bin Laden during the 1980s, The Washington Post said.
''I am with them,'' Atiyah writes to Zarqawi about the high command, ''and they have some comments about some of your circumstances,'' the article said.
In the letter, Atiyah spoke of 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 newspaper said.
According to the report, Atiyah's letter also shed new light on the depth of al Qaeda's concern over Zarqawi and the limits of its control over him.
It was the first document to emerge from what the military described as a ''treasure trove'' of information when Zarqawi was killed, the newspaper reported.
The letter 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.''