The bloodied children are clearly civilians; men killed as they launch rockets are undisputedly not. But what about the 40 or so young Hamas police recruits on parade who died in the first wave of Israel's bombing campaign in Gaza?

And weapons caches are clearly military sites – but what about the interior ministry, hit in a strike that killed two medical workers; or the money changer's office, destroyed last week injuring a boy living on the floor above?

As the death toll mounts in Gaza, the thorny question is arising of who and what can be considered a legitimate military target in a territory effectively governed by a group that many in the international community consider a terrorist organisation.

This is also the group that won the Palestinian legislative elections in January 2006 and a year later consolidated its control by force.

So while it was behind a campaign of suicide attacks in Israel and fires rockets indiscriminately over the border, it is also in charge of schools, hospitals, sewage works and power plants in Gaza.

International law

Israel says it is operating totally within humanitarian law, but human rights groups fear it is stretching the boundaries.

And as ground forces clash in the heavily-populated Gaza Strip, the questions will become more pressing.

International law’s rules on keeping civilian casualties to a minimum are based on the distinction between "combatants" and "non-combatants".

As Israel launched the first air strikes, outgoing Prime Minister Ehud Olmert said: "You - the citizens of Gaza - are not our enemies. Hamas, Jihad and the other terrorist organisations are your enemies, as they are our enemies."

But when an Israeli military spokesman also says things like "anything affiliated with Hamas is a legitimate target," things get complicated.

The International Committee of the Red Cross - guardian of the Geneva Conventions on which international humanitarian law is based - defines a combatant as a person "directly engaged in hostilities".

But Israeli Defence Forces spokesman Benjamin Rutland told the BBC: "Our definition is that anyone who is involved with terrorism within Hamas is a valid target. This ranges from the strictly military institutions and includes the political institutions that provide the logistical funding and human resources for the terrorist arm."

Philippe Sands, Professor of International Law at University College London, says he is not aware of any Western democracy having taken so broad a definition.

"Once you extend the definition of combatant in the way that IDF is apparently doing, you begin to associate individuals who are only indirectly or peripherally involved… it becomes an open-ended definition, which undermines the very object and purpose of the rules that are intended to be applied."

Indeed, Hamas itself has been quoted as saying the fact that most Israelis serve in the military justifies attacks on civilian areas.

Hamas policemen

The first wave of bombings, which targeted police stations across Gaza, is a key case in question - particularly the strike that killed at least 40 trainees on parade.

Analysts say Hamas policemen are responsible for quashing dissent and rooting out spies, as well as tackling crime and directing traffic.

But the Israeli human rights group B'Tselem, which has raised the issue in a letter to Israel’s attorney general, says it appears those killed were being trained in first aid, human rights and maintaining public order.

The IDF says it has intelligence that members of the police force often "moonlight" with rocket squads, but has given no details about the specific sites or individuals targeted.

However, campaign group Human Rights Watch (HRW) argues that even if police members do double as Hamas fighters, they can only be legally attacked when actually participating in military activities.

Both B’Tselem and HRW are also concerned about the targeting of ostensibly civilian sites such as a university, mosques and government buildings.

Protocol 1 of the Geneva Conventions - quoted by Israel, although not signed by it - says that for a site to be a legitimate military target it must "make an effective contribution to military action" and its destruction or neutralisation must also offer "a definite military advantage".

Israel says it has bombed mosques because they are used to store weapons, releasing video of the air strikes which it says shows secondary explosions as its proof.

But it gives no evidence for its claims that laboratories at the Islamic University, which it bombed heavily, were used for weapons research, or for its claims that at least three money changers targeted were involved in “the transfer of funds for terrorist activities”.

This is because Israel rarely releases intelligence material for fear of endangering the lives of its sources, Mr Rutland says.

However, on its targeting of the education, interior and foreign ministries and the parliament building, Israel simply argues they are part of the Hamas infrastructure – and there is no difference between its political and military wings.

"To claim that all of those offices are legitimate targets, just because they are affiliated with Hamas, is legally flawed and extremely problematic," says B’Tselem director Jessica Montell.

Questions of proportionality

Other incidents have raised concerns for these reasons, together with a second legal concept - proportionality.

This demands that the military gain of a particular operation be proportional to the likely or actual civilian losses incurred in carrying it out.

As Fred Abrahams, a senior researcher at Human Rights Watch puts it: "Even if you have a legitimate target you can’t just drop 10-tonne bombs on it."

Five sisters in the Balousha family were killed as they slept together as, apparently, a nearby Hamas-linked mosque was bombed in Jabaliya refugee camp on the second day of Operation Cast Lead.

HRW is calling for an investigation. "Was the mosque a legitimate target? We have our doubts… Did they use weaponry that would limit damage to civilians? We have our serious doubts," says Mr Abrahams.

In this case, Mr Rutland said the IDF had no record of a target in that specific area at that time, and gave no further explanation for the girls’ deaths.

A further case is the bombing of a truck that Israel initially said was loaded with missiles.

B’Tselem and the truck's owner – who said his son died along with seven other people – later said it was carrying oxygen canisters for welding. Israel maintains the warehouse the canisters were loaded from had been known to house weapons in the past.

How good was Israel's intelligence? How likely was it, for example, that at the moment of decision, the information might turn out to be wrong? And did the potential gains outweigh the possible losses?

Professor Sands says proportionality is "very, very difficult."

"What's proportionate in the eyes of one person may be disproportionate in the eyes of another," he says.

The difference in numbers in the Gaza war is stark - Palestinians say more than 500 Gazans have died in eight days, compared with 18 Israelis from rocket fire since 2001.

But experts say issues ranging from the parties' intentions, the reasons for going to war, the actions taken to protect - or indeed expose - civilians, and the conditions on the ground, all feed into a much more complicated legal equation.

Israel says lawyers are constantly consulted in its operations. It says it takes all possible steps to minimise civilian casualties.

Guided weapons are used; telephone warnings are often given before buildings are bombed; the IDF says missions have been aborted because civilians were seen at the target.

And it says its enemy is far from a standard army: "We're talking about an entire government whose entire raison d’etre is the defeat of Israel … and all of whose energies are directed at attacking Israeli civilians," says Mr Rutland.

Witnesses and analysts confirm that Hamas fires rockets from within populated civilian areas, and all sides agree that the movement flagrantly violates international law by targeting civilians with its rockets.

But while B’Tselem's Ms Montell describes the rocket fire as a "blatant war crime", she adds: "I certainly would not expect my government to act according to the standard Hamas has set for itself - we demand a higher standard."