The groups have granted safe passage to one another, shared recruits and coordinated their propaganda efforts, according to military officials.
They said that after a recent attack on a remote base in eastern Afghanistan they found evidence that the dead insurgents belonged to three different factions Mullah Omar's Taliban, the Haqqani network centred on Pakistan's North Waziristan region and Hezb-e Islami, led by Gulbuddin Hekmatyar.
Previously these groups were believed to share a common aim of driving international forces from Afghanistan, but were not thought to co-operate on missions.
Lieutenant Colonel John Dorrian, spokesman for the Nato-led military coalition in Afghanistan, said there was growing evidence of a "nexus" between the groups.
"There's a symbiotic relationship between the Pakistani and Afghan Taliban and the Haqqani network as well as al-Qaeda.
If they have someone with expertise in, for example, weapons or manufacturing [explosive devices], it's common for them to share that knowledge," he told The Daily Telegraph.
Earlier this month, the White House published a review of the war in Afghanistan which said the US was on track to begin withdrawing forces in 2011 following a surge of 30,000 troops.
Commanders with the Nato-led force in Afghanistan said the insurgents' new tactics reflected an attempt to wrest the initiative from American and allied forces.
"They have been forced to co-operate due to the effect our collective efforts have had on them," Lieutenant Colonel Patrick Seiber, a spokesman for American and coalition forces in eastern Afghanistan, told The New York Times.
(The Telegraph)