Donetsk, Ukraine: Was it Russian madness or a bolshie two-fingered salute to the world by separatist rebels in eastern Ukraine? Whichever, the downing of two Ukrainian jet fighters on Wednesday underscores the challenges in Canberra’s push for an international force to secure the MH17 crash site.

Dropping foreign forces into the region can’t be seen in isolation from a separatist war. And the intensity of the conflict has made quite an impression on Prime Minister Tony Abbott’s MH17 special envoy and former Air Chief Marshal Angus Houston.

Speaking after a ceremonial start to an airlift of the bodies of the victims of last Thursday’s shooting down of a Malaysia Airlines jet, Mr Houston said: “I’ve been here for just 60 hours and what strikes me is that this is a very serious war around the crash site.”

His point was well made when, within minutes, the two Ukrainian Su-25 fighter jets plummeted to earth, within 50 kilometres of the MH17 crash site.

The rebels claimed them as trophies of war and Kiev charged that the missiles that brought them down had been fired from Russian territory.

The downing of the jets also coincided with the first rebel admission that they have access to the Buk missile system, which Kiev and Western intelligence agencies say is what brought down the Malaysian Boeing 777, with 38 Australians citizens and residents among the 298 passengers and crew who died.

In a remarkable interview with Reuters, which he subsequently denied in an interview published in Russia, rebel leader Alexander Khodakovsky said that Russia might have provided the Buk system to his rebel factional opponents under the leadership of the Russian fighter Igor Strelkov.

Cutting across previous rebel denials, Khodakovsky is quoted: "That Buk I know about. I heard about it. I think they sent it back [to Russia]. Because I found out about it at exactly the moment that I found out that this tragedy had taken place. They probably sent it back in order to remove proof of its presence.”

The jets crashed near Dmitrievka, about 100 kilometres east of Donetsk and just 10 kilometres from the Russian border.

But the ground war is also becoming more intense as Kiev, despite its claims to have ceased anti-rebel offensives within 40 kilometres of the crash site, attempts to put a squeeze on the rebels.

Its efforts to corral rebel fighters near the border is reportedly meeting stiff resistance.

Kiev, nonetheless, is claiming success around this city, where it says that rebels are abandoning outlying positions and retreating into Donetsk, where there were reports on Wednesday of defensive trenches being dug near the university.

And military traffic in the region is heavy; driving from Kharkiv to Donetsk on Wednesday, Fairfax Media overtook several Ukrainian convoys, one of which included several trucks loaded with big artillery shells.

This then is the context in which the governments that Mr Houston describes as the MH17 "stakeholders" are proposing to establish a major civilian-based investigative effort that would be spread over 50 square kilometres, almost double earlier estimates, and would probably take months to conclude.

Investigators are assembling in Kiev and a few have made brief, rebel-supervised visits to the crash site.

But before there can be a serious investigative presence, the site has to be secured, Mr Houston said.

“We have to work with the Ukrainian government and our partners - there’s a number of ways to secure the site and we need smart military and police minds in the capitals of the stakeholders, working professionally and unemotionally [because] we’ll not get the work done that is required to be done unless we have adequate security.”

Seeming to allude to Dutch revelations that as many as 100 bodies could have been missing from the train that was thought to have delivered virtually all the remains of the 298 who died to the northern city of Kharkiv on Tuesday, Mr Houston spoke of a need to scour the entire site for debris, adding: “and hopefully we’ll find some things that we didn’t expect to find”.

Like the Dutch leader of the investigation, Mr Houston vowed that no human remains could be left behind, saying: ‘This job is vital.”

But then tracing the arc of an even more ambitious operation, he said: “We must recover all body parts and we need to go over the whole site and get every personal effect - we owe this to the families.

“Also, every last bit of wreckage - each bit, you know, indicates something to the eye of a trained investigator and hopefully it will produce evidence that’ll stand up in court and that it will lead to people being held accountable.”

Asked about Kiev stepping up its campaign against the rebels, Mr Houston observed that there were "major operations" to the north of the crash site. “If you can hear mortar fire, it means that it’s not very far away.”

So what was the Ukrainian government reaction to the Australian Prime Minister’s proposal for an international force, an issue on which Ukraine’s deputy Prime Minister Volodymyr Groysman twice ducked Fairfax Media’s questions this week.

Mr Houston: “We’re talking ... it’s in the very early stages ... and it's going to be very difficult to work out how [security] will be provided.”

Was all this a diplomatic way of saying that Kiev was pushing back on the Abbott proposal - ‘it’s too early to say that.”

Kiev is deeply committed to what it calls an anti-terrorist operation against the rebels - and some of the foreign capitals that might be asked to join an MH17 force, would most likely want Kiev to suspend its campaign against the rebels.

Mr Houston did observe that Kiev was capable of delegating - it did surrender leadership of the crash investigation to the Dutch.

On this Mr Abbott’s envoy is right - up to a point. It is one thing for Kiev to delegate the investigation to others, a tiresome business for which it doesn’t have the resources and in which it would have difficulty meeting the forensic standards demanded by other countries.

But accepting a foreign military presence on its own soil might be a bridge too far.

What would it say to the world of Kiev’s capacity to secure its territory and to defend its people? The rebels already are proving Kiev’s incapacity in this regard and to accept a foreign force might be to rub too much salt in that wound.

And if Kiev could be coaxed to accept such a force, there would then be a challenge of a different kind for the foreign governments: holding it back from goading the rebels to strike, to provoke clashes in which the Ukrainian government would then demand that the foreigners help them to see off the rebels.

Security in a war zone? It’s a tricky business.