Sitting next to him, Abdallah Suleiman Omary, an engineer with a stubbly beard and reading glasses, draws a map of the al-Ghazaliya area of Baghdad, where he was living until recently, in an attempt to explain the impact of the recent US "surge" in troop numbers on areas where Sunni resistance is strongest. "They have now built a concrete wall blocking all but one road into the district from the Baghdad expressway," he says. "To get to my house, you have to pass a bridge with a checkpoint, an American base, a gated entrance and a further four checkpoints in one kilometre. Yesterday, they banned all cars in the area because of the fear of car bombs.

"But," he adds, "we are still able to launch attacks. Weapons are brought in by hand. Fighters watch the soldiers until they leave the checkpoints to buy something - then they follow them and kill them."

Both men are leaders of the Iraqi resistance - or insurgency, as it is usually known in Britain and the US. Zubeidy is the political spokesman of Ansar al-Sunna, an Islamist armed group with a ferocious reputation in Iraq, and Omary is head of the political department of the 1920 Revolution Brigades, a more nationalist organisation whose name commemorates an uprising against British rule after the first world war.

For four years, the resistance has stayed in the shadows, without a public face and apparently leaderless, while delivering an ever more violent and devastating campaign that has brought the world's most powerful army to the brink of defeat and changed the balance of global power. As al-Qaida-style suicide atrocities against civilians and Sunni-Shia sectarian death-squad killings have escalated in the past couple of years, they have tended to shift attention away from the guerrilla war against the US and British occupation forces and their client Iraqi army and police. But it is that growing war of attrition - there are now more than 5,000 attacks a month against US forces across Iraq and the past three months have been the bloodiest for US forces since the 2003 invasion (331 deaths and 2,029 wounded) - that has pushed the demand for withdrawal from Iraq to the top of the political agenda in Washington.

Until now, the resistance groups have operated entirely underground and their leaders have communicated with the outside world mainly through internet postings, if at all. (Omary's group specialises in hi-tech communication and produces photos and videos, some of them reproduced here, which are strongly reminiscent of IRA propaganda of the 1980s.) Now they have decided to speak to the western press for the first time as they prepare to launch a public face and a common political programme in anticipation of eventual American and British withdrawal from Iraq. Seven of the most important Sunni-led armed organisations - excluding al-Qaida and the Ba'athists - have agreed to form a united front and have drawn up a series of demands to form the basis of future negotiations with the occupation forces.

Our meeting takes place in Damascus, arranged through a series of intermediaries. As well as Zubeidy and Omary, one of the political leaders of Iraqi Hamas - a trader from Fallujah who uses the nom de guerre Abu Ahmad - also takes part in a conversation that lasts for nine hours. All three have come recently from Iraq and are wary and uncertain of how open to be in this interview as they edge towards a public profile for the Iraqi resistance. These are the kinds of insurgent leaders who - according to Dan Goure, vice-president of the Virginia-based Lexington Institute and a Pentagon consultant - the US military intelligence is increasingly targeting in the hope of taking out the leadership of what the resistance leaders themselves estimate as a 50,000-strong movement.

At least 1.2 million Iraqis have sought refuge in Syria since the US invasion - the equivalent of about 5 million refugees in Britain - and Syria is regularly accused by the US of supporting or turning a blind eye to insurgent groups operating across the border. Zubeidy insists that they have no direct contact with the Syrian regime - and they certainly don't advertise their presence in Damascus. "Our organisations depend on stores of weapons left by Saddam Hussein, or we buy them from the Iraqi army through merchants," says Zubeidy. "We try to avoid links with the Syrians. Either they can sell us out at any time if there is heavy pressure on them, or we risk being completely controlled by them."

Indeed, all the groups say they have no support from any regime, although they do claim that there has been indirect contact with France about creating the conditions for a public office. And, a couple of years back, Zubeidy says, Iran offered the Islamic resistance groups weapons, money and also help with stopping attacks from the Shia militia, but while he believes al-Qaida accepted, the others did not. "We do not trust Iran. We need help from Arab and other governments. But although Turkey and Saudi Arabia have encouraged the resistance to unite, they are afraid of us".

"We are the only resistance movement in modern history that has received no help or support from any other country," Omary declares. "The reason is that we are fighting America." The 1920 Revolution Brigades spokesman is an articulate and sophisticated operator, who - if he survives the counterinsurgency and sectarian onslaught - clearly has the potential to become an influential voice in a future Iraq. "Our position is that there are two kinds of people in Iraq: not Sunni and Shia, Kurdish and Arab, Muslim and Christian, but those who are with the occupation and those who are against it." Anyone who takes part in the institutions set up by the occupation, such as the government and parliament, army or police, are regarded as collaborators. "Our organisation began its operations in the first days after the invasion and wherever you find the occupation, you will find us: from Mosul, Baghdad and Samarra to Basra, Hillah and Kirkuk," continues Omary. "Our group has also carried out attacks on British forces in Basra." They are not a Sunni sectarian organisation, he insists: "The military leader of the Brigades is a Kurd. Iraq is for all Iraqis and we only distinguish between those who cooperate with the occupation and those who do not. If my brother cooperates with the occupation, I will kill him - but the innocent must not be touched."

What makes Iraqis join the resistance? "Many people come to the resistance because of their Islamic background, some because of what has happened to their relatives at the hands of the occupation armies," says Zubeidy. "American forces have committed very big crimes against the Iraqi people. All Iraqis hate the foreign forces and won't forget what they have done. Generally, British forces have acted as a helper to the US and the British government shares the blame for everything that happened to Iraq. But their actions are seen as having been less cruel than the Americans."

At the heart of the new insurgent alliance is a rejection of the murderous sectarianism that has come to grip Iraq - and the role of al-Qaida in particular. Most striking is the case of Zubeidy, whose hardline salafist (purist Islamic) group Ansar al-Sunna recently split in half over the issue (his faction is now called the Legitimate Committee of Ansar al-Sunna - Goure says such splits are endemic in the resistance movement). "We wanted to unite with other resistance forces, but the other group is moving closer to al-Qaida and refused. Al-Qaida has brought benefits and problems," Zubeidy says. "They attack the US occupiers. But every day the problems they bring become greater than the benefits.

"Resistance isn't just about killing Americans without any aims or goals," he continues. "Our people have come to hate al-Qaida, which gives the impression to the outside world that the resistance in Iraq are terrorists. Suicide bombing is not the best way to fight because it kills innocent civilians. We are against indiscriminate killing - fighting should be concentrated only on the enemy. They [al-Qaida] believe that all Shia are kuffar [unbelievers] - and most of the Sunnis as well." They estimate that al-Qaida now carries out between a fifth and a third of all attacks in Iraq.

But they say that it is necessary for the Sunni-based groups to ally with the Shia. "Even though that is not easy," says Zubeidy. "A great gap has opened up between Sunni and Shia under the occupation and al-Qaida has contributed to that - as have the US and Iran. Most of al-Qaida's members are Iraqis but its leaders are mostly foreigners. The Americans magnify their role, even though they are responsible for a minority of resistance operations - remember that the Americans brought al-Qaida to Iraq."

Sectarian division has been inflamed, Omary adds, as part of the "old British imperial tactic of divide and rule". Who benefits, asks Abu Ahmad, when there are bombs in markets? "It is only the occupying forces and Iran." All the Sunni-led resistance groups are acutely aware of the threat posed by sectarian division to the future of Iraq and emphasise their strong links to those Shia with fewer links to Iran (what they call Arab "national" Shia), but reject any suggestion of making common cause with the Shia militia and political parties - including the anti-American Mahdi Army, which is thought to be behind many of the current armed attacks on British forces in Basra - because of their participation in the occupation's political institutions and their role in ethnic cleansing and sectarian killings. The man who came to Zubeidy's house last year and killed his brother-in-law after demanding a million-dollar ransom claimed to be from the Mahdi Army.

To emphasise their own distance from the kind of atrocities that have become associated in the western imagination with the Iraqi insurgency, Omary mentions the case of Wassef Hassoun, a Muslim Lebanese-American US marine captured by the 1920 Revolution Brigades and released after he swore an oath on the Koran that he would not continue to fight with US forces. "Our people took him from the marine base outside Falluja - they got into the base in a rubbish lorry and took him out with them. They let him go after he pledged not to go back to his unit. We did something similar with an Egyptian lorry driver working for a transport company under contract to the Americans." Hassoun was later charged with desertion after he failed to return from leave after his release. He is still missing.

During breaks in the conversation, Zubeidy stops to pray, Abu Ahmad fingers his worry beads and Omary shows me a video clip on his mobile phone of the body of a neighbour's 12-year-old child, Mustafa, with a huge wound in his chest, laid out for mourning in the family home. "He was shot last month by an Iraqi soldier 200 metres from an American base, apparently for fun. The same night, resistance fighters went to the Iraqi soldiers' billet and killed all of them in the house, half a dozen of them, by cutting their throats." While all seven of the armed groups joining the new front reject attacks on civilians, they have no qualms about brutal violence against Iraqi police and soldiers, or US and British troops. But aren't there other, more peaceful ways to try to achieve their aims?

"Peaceful resistance will not end the occupation," states Abu Ahmad. "The US has made clear that it intends to stay in Iraq for many decades. Now it is a common view in the resistance that they will start to withdraw within a year." Right or wrong, that is one of the factors that has led to the decision to form the new front, which is planned to be called the Political Office for the Iraqi Resistance. As well as Iraqi Hamas, the 1920 Revolution Brigades and the new Ansar al-Sunna, it is to include the powerful Jaish (army) al-Islami, Jaish al-Mujahideen, Jama' and Jaish al-Rashideen. The plan is to hold a congress of the seven groups to announce the front's formation and then move towards the establishment of some form of public presence outside Iraq, though it is hard to see any state being prepared to risk the wrath of the US by hosting such an outfit. "It would need UN protection," Zubeidy suggests.

They have already agreed that at the heart of the programme will be a commitment to liberate Iraq from all foreign troops; recognise only those who reject the occupation and its institutions as able to represent Iraq; demand compensation from foreign forces for the devastation they have inflicted on the country; declare all decisions taken by the occupying states and its client government null and void; and reject any change in population distribution. The aim is for the front to join other independent anti-occupation forces from across the country to negotiate with the Americans for their withdrawal. A temporary technocratic government would then manage the country during a transition period until free elections could be held for a new independent government. Even Saddam's revamped Ba'ath party - which now plays what is regarded as a reduced role in the resistance - is an enthusiast for fully competitive elections.

But what if the US doesn't start to withdraw from Iraq next year, as the resistance groups expect, or merely withdraws to the huge military bases it has built around Iraq to intervene as and when it sees fit? "As long as foreign forces remain in Iraq," Omary replies, "the Iraqi government will not be independent. And armed resistance will continue."

· Names have been changed.