By David Bamford
BBC security correspondent
A statement has appeared on an internet website used by a militant Islamic group in Iraq, declaring allegiance to al-Qaeda and Osama Bin Laden.
Zarqawi's group has claimed a string of attacks in Iraq
The group, Tawhid and Jihad, is led by Jordanian Abu Musab al-Zarqawi.
He is now seen by the US as a bigger threat than Bin Laden following attacks in Iraq carried out in his name.
There has been speculation before now about whether Zarqawi and Bin Laden are allies or rivals for control of the Islamist militant movement.
Some reports claim the two men have little connection at all.
"We announce that Tawhid and Jihad, its prince and its soldiers, have pledged allegiance to the leader of the mujahideen, Osama Bin Laden," the statement said.
It added that the pledge of allegiance was to coincide with the start of the Islamic fasting month of Ramadan because Tawhid and Jihad agreed that Muslims must "ally together in the face of its enemies".
The statement said that Zarqawi had previously been in contact with Bin Laden's movement for eight months but communications had then been broken. The statement did not say why this had happened.
"After contact was restored, the al-Qaeda leadership understood our strategy in Iraq," the statement said.
The authenticity of the statement has not been confirmed.
Mystery surrounds the fate and whereabouts of Osama Bin Laden
In the past, bogus messages have been posted to Islamic websites in the name of Tawhid. But several analysts said this statement - which would be Tawhid's first acknowledgement of loyalty to al-Qaeda - may well be genuine.
Some experts suggested it was part of a new Tawhid strategy to encourage more recruits to join in the insurgency in Iraq, by associating it with the wider struggle for US defeat and withdrawal from the Middle East, and for a wider Islamic revolution.
Washington has long held that Bin Laden, Zarqawi and Islamist insurgency movements stretching from Morocco to Indonesia are part of the same phenomenon under the al-Qaeda label.
The Bush administration, reacting to the internet statement attributed to Tawhid, said that while it too was still trying to confirm its authenticity, the US has always assumed Zarqawi had close ties with al-Qaeda.
"This underscores once again why Iraq is the central front in the war on terror," White House spokesman Trent Duffy said in Washington.
"It's also proof positive of why the president's firm resolve to fight terrorists overseas so we don't face them in America's neighbourhoods is the only clear way to prevail."
But others have suggested that the two men have acted more like rivals than allies.
Until now, audio and video statements issued by Bin Laden's lieutenants referred to the struggle in Iraq without referring to Zarqawi specifically.
Similarly, Tawhid has referred only obliquely to al-Qaeda.
If this internet statement is a genuine communique, Tawhid has now formally declared its allegiance to Bin Laden, which it describes as the "chief of all fighters".
Zarqawi, believed to be operating from Falluja or elsewhere in Iraq, has gained notoriety because of the devastating attacks and hostage beheadings blamed on - and claimed by - Tawhid and Jihad.
Bin Laden, assumed to be in hiding - possibly in Pakistan - with his deputy Ayman Al-Zawahri, remains a powerful figurehead for those favouring global Islamic revolution or, as some analysts believe, a global resurgence of Arab influence on the back of the call to Islam.
Zarqawi and Bin Laden both rose to prominence as "Afghan Arabs" - ethnic Arabs who went to Afghanistan in the 1980s to join the declared jihad (holy war) against the occupying Soviet forces.
Zarqawi returned to carry out attacks in his native Jordan aimed at ending US influence and destroying the Hashemite monarchy.
Imprisoned in Jordan and later fleeing abroad, he returned to Afghanistan and established his own training camps separate from those of Bin Laden.
Bin Laden's stated main aims in the 1990s were to force the US out of the entire Arabian peninsular and bring down the Saudi monarchy.
The 11 September 2001 attack in the US, blamed on al-Qaeda, gave Bin Laden worldwide notoriety.
Following the US-led invasion of Iraq two years later as part of President Bush's declared "war on terrorism", Zarqawi emerged as the leader of the strongest group of insurgents fighting the US-led coalition forces.