The BBC's security correspondent, Frank Gardner, looks at who might be behind suicide and other attacks on coalition troops and civilians in Iraq.
Big car bomb attacks are generally well planned and executed
Q: Who is behind the continuing attacks in Iraq?
We have to distinguish here between the frequent, low-level hit-and-run attacks on US forces on the one hand and the far more sinister suicide car bombings which have killed so many civilians, on the other.
Those behind the roadside bombings, the snipings and the RPG attacks on coalition troops are thought to be a mixture of Saddam loyalists, untrained Arab volunteers and ordinary Iraqis with a score to settle with the US military which may have killed members of their family during the war.
Those behind the big car bomb attacks are believed to include al-Qaeda affiliates, possibly working in collaboration with members of the former regime.
Q: How much do we know about foreign involvement?
The question of foreign involvement is murky and so far inconclusive.
It is known that just before, and during, the Iraq war hundreds of Arab "volunteers" - some calling themselves mujahideen or "holy warriors" - filtered across the border into Iraq. They came from a range of countries - Yemen, Saudi Arabia, Lebanon and North Africa.
Most of these men were either killed or returned home, dispirited at how quickly the Iraqi army collapsed. Some remained and they have been joined by a fresh wave who have found it easy to slip into Iraq.
Judging by the identity papers found on some of those killed by US troops, Arab volunteers are actively volunteering for attacks on coalition forces and Iraqis working for them. The US military, for example, has reported finding a Syrian passport on a man who tried unsuccessfully to blow up a Baghdad police station.
Q: Do any of the tactics or techniques bear recognisable hallmarks - for example, of al-Qaeda?
They do. The big car bombs on non-military targets have borne the hallmarks of al-Qaeda.
They have been professionally planned and executed, sometimes synchronised, usually carried out by suicide bombers and aimed at maximum publicity effect. They have inflicted heavy and indiscriminate civilian casualties and no-one has claimed responsibility.
Q: Are their methods getting more sophisticated - and more effective?
Yes. The death tolls from these bombings are rising. The attackers seem to be able to overcome the safety measures put in place by the coalition.
The attack on Baghdad's Rashid Hotel on 26 October was an example of their ingenuity - although this attack on a coalition hotel could well have been the work of Saddam loyalists.
The coalition had hoped some of their top officials would be relatively safe, protected by a cordon of concrete walls. But they were attacked by rockets fired over the walls from 400 metres away from a trailer disguised as a generator. The attack on Red Cross headquarters in Baghdad on 27 October apparently used a bomb hidden inside an ambulance.
Q: Where do they get the weapons from?
Iraq is awash with weapons so nobody needs to import them.
Q: How are they picking their targets?
For anyone wishing to undermine the coalition's work, Iraq is what could be called a "target-rich environment".
With close to 150,000 troops in the country, the Pentagon's soldiers make easy pickings for those planting mines and roadside bombs. They know which routes are taken by convoys and they are often gone by the time the Americans are able to react.
The bigger targets are chosen more carefully, almost certainly with advice and possibly direction from former Baath party members.
The attacks on the UN headquarters, the ICRC, the Jordanian and Turkish embassies, as well as attacks on ministries and Iraqi officials co-operating with the coalition, are all designed to make Iraq ungovernable.
The attackers want nobody to help the Americans, for the place to descend into chaos. The Saddam-al-Qaeda link was never proven before the war, but in this new phase they do have something in common.
The Saddam loyalists still dream of revenge and a return to the status quo before the war. Al-Qaeda's affiliates want to see the US-led invasion and occupation of an Arab and largely Muslim country fail and for the Americans to be driven out - just as they were in Lebanon in 1983 and again in Somalia 10 years later.
Q: How much support do they have among the wider Iraqi population?
It is hard to gauge as it depends whom you talk to. In the south around Basra, Saddam was universally loathed. Before the attack on the Italian peacekeepers' HQ in Nasiriya on 12 November, attacks were minor, mainly carried out by criminal elements linked to smugglers, thieves and those with old scores to settle with other Iraqis.
In the north there is little evidence of any Kurdish resistance to the coalition presence - so far events have largely suited the Kurds. But in the centre and to the immediate west and northwest of Baghdad there are pockets of diehard opposition to the Americans centred on the towns of Falluja, Ramadi, Baquba and Tikrit.
Money undoubtedly plays an important role. The US dollar bounty allegedly paid by Saddam loyalists to those willing to attack the coalition is said to be rising.
Q: How much of a threat do they pose to the American forces?
Despite what some in Washington say, Iraq is turning into a protracted guerrilla war. Americans are dying almost daily and if that continues there will come a time when the political price for President Bush may become too high.
There are just so many US troops in Iraq that the Pentagon cannot protect them all. In order to do the job of stabilising security in Iraq they have to come out of their bases and go on patrol. Since their attackers look exactly the same as ordinary civilians it is relatively simple for them to be picked off in small numbers by roadside bombs, concealed mines and hit-and-run attacks with mortars and RPGs.
The only way the US can counter this is by getting better intelligence from their Iraqi sources.