By Jonathan Marcus BBC defence correspondent

US commanders feel pacifying Iraq may take longer than expected

The rash of attacks raises a number of fundamental questions about post-war security operations, with some analysts speaking of signs of a growing guerrilla war against the US occupation.

There is no doubt that attacks against US forces in particular seem to be mounting.

British troops, too, have had a bad week, with six men killed and eight injured in two separate incidents on Tuesday.

Undermining efforts

Opposition in Iraq comes from many sources. Baath Party loyalists, Iraqi nationalists who oppose the invasion of their country, criminal groups and local war-lords and militias are all trying to carve out and protect their own turf.

As the experience of the British troops indicates, even relatively benign areas can suddenly turn dangerous

Foreign fighters may also be an element in the mix.

To speak of a full-scale guerrilla war may be premature, but this rumbling, low-level conflict - from whatever source - has a cumulative impact. It undermines US and British efforts to restore normality.

One of the most worrying recent developments is attacks against Iraqi engineers and others associated with attempts to get the infrastructure of the country up and running.

Elusive targets

By definition the threat facing US and British forces is diffuse.

Establishing a secure climate in Iraq is proving harder than anticipated

Large-scale counter-insurgency operations are problematic.

Often they tend to miss their elusive targets and simply serve to upset local people over a wide area.

As the experience of the British troops indicates, even relatively benign areas can suddenly turn dangerous.

This highlights the fundamental problem of local intelligence gathering, inevitably complicated by differences of language, religion and culture.

Poor planning?

Senior US commanders are already signalling that pacifying Iraq is going to take longer than expected.

Rather than replacing tyranny with order, tyranny was replaced by a power vacuum in many places

Much of the problem stems from the way in which the war ended - indecisive in the sense that the top regime leadership apparently escaped and large parts of the Baathist machine simply drifted away. Now some are emerging to fight another day.

It is easy to be negative.

Much has been achieved and post-war Iraq was never going to be an easy place to sort out.

But there are concerns that the Americans, in particular, planned poorly for the immediate aftermath of the conflict.

There almost certainly were too few troops to impose order - a fact that military planners should ponder when talking about moving towards smaller, technology-intensive units to fight high intensity battles.

An opportunity may have been missed.

Rather than replacing tyranny with order, tyranny was replaced by a power vacuum in many places.

That does not necessarily mean that everything is turning into a disaster.