Canadian-born jurist Louise Arbour, the UN's high commissioner for human rights, has once again put the international community on notice by suggesting that some military tactics being used in Lebanon might lead to prosecutions for war crimes.

Arbour did not single out either Israel, which has been raining bombs on Lebanese targets, or the Hezbollah militia, which has been firing rockets indiscriminately into northern Israel.

In a statement from her Geneva office she said: "The scale of killings in the region, and their predictability, could engage the personal criminal responsibility of those involved, particularly those in a position of command and control."

Noting that international humanitarian law as well as international criminal law contain obligations to protect civilians during conflict, she said: "Indiscriminate shelling of cities constitutes a foreseeable and unacceptable targeting of civilians. Similarly, the bombardment of sites with alleged innocent civilians is unjustifiable."

This is not the first time that Arbour has prodded the international community on the issue of war crimes. The former chief prosecutor for war crimes in the former Yugoslavia (and Rwanda) at the International Criminal Tribunal in The Hague, Arbour made legal history by being the first to indict a sitting head of government, Serbian strongman Slobodan Milosevic.

• INDEPTH: War crimes

Last fall, in her new role as the UN's top human rights official, she angered the Bush administration by publicly challenging its definition of torture and its handling of Iraqi and other terror detainees in accordance with the Geneva Conventions.

The first hurdle: Jurisdiction

But the rules for what constitutes a war crime are not as clearcut as might appear at first glance, and as there have been few complete prosecutions in the modern era (Milosevic died during his trial), there are few precedents to draw on.

What's more, the big hurdle in this case would be whether the International Criminal Court, which was established in 2002 as the world's permanent war crimes tribunal, has jurisdiction. Neither Israel nor Lebanon, for their own reasons, has ratified the ICC treaty, and Hezbollah, as an independent group, would seem outside its reach.

These difficulties could probably be overcome if the UN decided to set up its own prosecution over alleged war crimes in Lebanon, as has been done in the past, or referred the matter to the ICC.

But this approach would likely be subject to the unanimous support of the Security Council members, and it is difficult to see that happening given the current split between the U.S. and other Security Council members over the appropriate use of force in this crisis.

Complicating the referral option is that almost no Arab country has ratified the ICC.

The U.S., as well, has not ratified the ICC treaty and says it won't do so because it fears politically motivated prosecutions.

Civilian casualties

Arbour's contention is that the indiscriminate shelling of urban centres can constitute an "unacceptable targeting of civilians." And she's right, of course. But the devil's in the details.

The 1949 Geneva Conventions, seeking to outlaw the Second World War bombing raids against largely defenceless cities, state that civilian populations and individual civilians shall not be the object of an attack. And a Vietnam-era addition to the protocols enjoins military leaders to take constant care to avoid civilian casualties.

Does this care extend to civilian infrastructure like roads, airports or even apartment buildings or suburban backyards from which missiles may have been launched?

The Conventions are more obtuse on this front. They say military targets are limited to those things that "make an effective contribution to military action," which can be read as a pretty big loophole.

Carpet bombing is clearly prohibited. But is the indiscriminate bombing of civilian centres, like the 1,200 or so unguided small-rocket attacks Hezbollah has been launching nightly at Israeli towns, a breach of the Geneva rules? Many would say yes, though Hezbollah would argue self-defence in the face of an overwhelmingly superior enemy force.

In his book Crimes of War, German war crimes expert Horst Fischer argues that the Scud missile attacks on Israel by Iraq during the Gulf War in the early 1990s amounted to a breach of the Geneva Conventions because they were known to be widely inaccurate and there was no indication Iraq took care to aim them at military targets.

In Fischer's view, terror attacks aimed at civilians and cluster bombs set off in city centres are clearly war crimes.

Proportionality

Has Israel reacted with excessive force in destroying much of Lebanon's infrastructure and killing more than 300 civilians in the first five days of fighting, as compared with about a tenth that number of Israelis who lost their lives?

Many say yes, from the presidents of Lebanon and France to the International Red Cross in Geneva who are all urging a ceasefire. But calling the action excessive is not the same as calling it a war crime.

States cannot take unlimited, overpowering action against a transgressor; neither can individuals fending off intruders. The principle of proportionality is embedded in almost every legal system in the world, notes Fischer.

In the late 1990s, when NATO authorized something like 25,000 missile strikes and bombing raids against warring parties in the former Yugoslavia, several people, including a Canadian law professor, filed complaints with the International Criminal Tribunal, wanting it to prosecute NATO officials for war crimes. Up to 1,800 civilians were killed by these attacks.

The tribunal appeared to consider the idea (Arbour was just leaving the organization at the time), but in the end did nothing but muse about it publicly.

The key issues here are that proportionality requires a measured response and a continuing obligation on the part of military commanders to anticipate and be updated on civilian movements.

As well, some of the states that ratified the 1977 protocol on the incidental loss of civilian life said that such incidents can only be considered as part of a broader chain of events, and not just from an isolated occurrence.

Hostages

The taking of hostages, which both Hezbollah and Hamas did at the outset of this particular conflict, is directly prohibited under Common Article 3 of the Geneva Conventions, as well as the slightly looser customary international law involving armed conflicts, which should apply here.

But does this convention also apply to the score or so of Hamas legislators the Israeli army rounded up in response and plunked in a detention centre? Are they hostages or not? It is not clear what authority Israel was using to make the arrests.

Targeting and capturing enemy soldiers is allowed under international law. But once captured, these soldiers must be treated humanely and can't be used as hostages.

What is also clearly a war crime is summarily executing anyone held in detention, though whether that will be enough of a deterrent to save the lives of the three Israeli soldiers currently being held by Hezbollah and Hamas is anyone's guess.

The militant groups are still calling on Israel to stop its attacks and negotiate an exchange for some of the 9,000 or so Arab prisoners in Israeli jails.

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