Documents found in the house where Osama bin Laden was killed a year ago show a close working relationship between top al Qaeda leaders and Mullah Omar, the overall commander of the Taliban, including frequent discussions of joint operations against Nato forces in Afghanistan, the Afghan government and targets in Pakistan.
The communications show a three-way conversation between Bin Laden, his then deputy Ayman Zawahiri and Omar, who is believed to have been in Pakistan since fleeing Afghanistan after the collapse of his regime in 2001.
They indicate a "very considerable degree of ideological convergence", a Washington-based source familiar with the documents said.
The news will undermine hopes of a negotiated peace in Afghanistan, where the key debate among analysts and policymakers is whether the Taliban - seen by many as following an Afghan nationalist agenda - might once again offer a safe haven to al-Qaida or like-minded militants, or whether they can be persuaded to renounce terrorism.
One possibility, experts say, is that although Omar built a strong relationship with Bin Laden and Zawahiri, other senior Taliban commanders see close alliance or co-operation with al-Qaeda as deeply problematic.
Western intelligence officials estimate that there are less than 100 al Qaeda-linked fighters in Afghanistan, and last year the United Nations split its sanctions list to separate the Taliban and al-Qaida.
Both David Cameron and US secretary of state Hillary Clinton have said that some kind of political settlement involving the Taliban is key to the stability of Afghanistan once most western troops have withdrawn by 2014.
Some communications in the documents date back several years but others are said to be from only weeks before the raid on 2 May last year in which Osama Bin Laden died.
"Questions and issues come up. They don't see eye to eye on everything but it's clear they understand they have an interest in co-operating (on attacks against Nato, Afghan government and Pakistani targets)," the source said.