Eyes are on the skies in Ukraine, with the country's air defences becoming all the more critical as Russia continues to hammer cities and energy infrastructure with a barrage of missile strikes.
Over the past two weeks, Moscow has unleashed its largest wave of missile and kamikaze drone attacks since the start of the nine-month war.
A senior Pentagon official has cautioned that Moscow is attempting to deplete Ukrainian air defences which have so far prevented the Russian military from establishing dominance of the skies above the country.
Western allies, particularly across Europe, have been clearing out old warehouses and sending air-defence systems still lying around from the Cold War to boost Ukraine's coverage.
And earlier this month, the country received its first delivery of the long-awaited National Advanced Surface-to-Air Missile Systems, or NASAMS, from the US.
"Look who's here! NASAMS and Aspide air defence systems arrived in Ukraine!" Ukraine's Defence Minister Oleksii Reznikov tweeted.
"These weapons will significantly strengthen #UAarmy and will make our skies safer."
President Volodymyr Zelenskyy said NASAMS were responsible for shooting down "more than 70 missiles and 10 attack drones" in a volley of strikes that resulted in the deadly spillover into Poland.
But experts say the invasion has become a game of numbers in the "defensive-offensive tango", and there are issues arising as the war stretches on.
Even sophisticated systems like NASAMS are proving to have supply and capability limitations.
So where does Ukraine's air defence stand, and what more can actually be done?
NASAMS a 'step up', but they're not enough
The US sped up the delivery of the first two NASAMS and has promised six more.
Jointly produced by the US and Norway, the medium-range air-defence system uses a combination of sensors and radars to detect and defend against cruise missiles, aircraft and drones.
It includes a mobile radar system, a control-and-command centre where soldiers can monitor threats, and a separate launcher.
The trailer-based system can be easily moved to different locations.
US Defense Secretary Lloyd Austin has claimed that NASAMS have a 100 per cent success rate in intercepting Russian missiles.
Although they only have a maximum target range of around 30 kilometres, Michael Shoebridge, director of Strategic Analysis Australia, says they are already having a "pretty decisive effect on the battlefield".
"For Ukrainians, this is an absolute step up in their defensive capability," he said.
A big advantage of NASAMS is that they use standard air-defence missiles, including the Advanced Medium-Range Air-to-Air Missile (AMRAAM)
Professor Stephan Fruehling from the ANU Strategic and Defence Studies Centre said there was no risk NASAMS would run out of missile supplies.
"The advantage here is that there's literally thousands of those sitting in stores," he told the ABC.
"Most ground-based air-defence [systems] use specialised, dedicated new interceptors."
The downside is that they were not intended as a wide-area defence system, so what Ukraine can defend is limited.
They were originally designed to defend air bases and are currently used to protect sensitive airspace, including around the White House.
"They're like footprints around various locations," Mr Shoebridge said.
Only sites at major cities, and key targets such as energy infrastructure, are being protected.
"Ukraine is a huge country — you need an awful lot of those kinds of systems," Professor Fruehling said.
"The fact is they just don't have enough to defend every important target."
And, it's unlikely they ever will.
Like many of the weapons and munitions being used in the war, there are production and supply issues — there are just not enough NASAMS available.
The system is currently being operated in around 12 countries, and Australia is among the list of nations which has them on order.
Although the US has promised to send more NASAMS to Ukraine, the Pentagon is yet to specify exactly when Kyiv can expect them.
What else is being used?
The Ukrainians have been doing a "pretty good job" with their air defence until now, Mr Shoebridge says.
They have been able to keep the Russian Air Force and its large fleet of bombers outside Ukraine.
Ukraine's air defence has largely been based on old Soviet devices, including the S-300 long-range surface-to-air missile system.
Russia also uses S-300s, which have a maximum standard range of 150 kilometres.
The S-300 is the only air-defence system Ukrainians have that is effective for high altitudes.
But, there is limited stock for S-300s, and the missiles are not produced by the West.
"So there is a real risk that Ukrainians are just running out of air-defence missiles," Professor Fruehling said.
"Since around summer, there's been a change in the sense that the West has realised that Ukraine needs to move from Soviet munitions and systems to Western munitions so there can be a steady supply."
'A whole potpourri' of systems
Ukraine still needs S-300s so Russian planes can't fly over the "engagement envelope" of NASAMS and other shorter-range systems. That is, the space within which a system can engage an incoming target.
The West has recently been sending more alternatives to help Ukraine cope with much higher usage rates of missiles and take the pressure off S-300 stocks.
Norway and Spain have sent older US-made HAWKs, and Germany has delivered the first of four high-tech IRIS-T air-defence missile systems.
In October, France said Crotale air-defence systems would be operational within two months, and promised to train 2,000 Ukrainian soldiers.
Italy indicated earlier this month that it was ready to provide Ukraine with a variety of air-defence systems, including the medium-range Franco-Italian SAMP/T.
Meanwhile, Lithuania has sent a stockpile of new anti-drone rifles called SkyWipers, which fry the electrodes of the high volume of cheap drones on the battlefield which have the potential to cause "big problems".
"There's a whole potpourri of similar systems to NASAMS going into Ukraine at the moment, but most of those are in a range of around 15 to 30 kilometres," Professor Fruehling said.
"There aren't actually that many air defence systems that have a very long range, largely because the West didn't really need any such system over the last six years."
Kyiv has been pushing for more advanced systems like the Patriot, which has a range of up to 70 kilometres.
The American built surface-to-air missile system takes longer to manufacture and is more expensive due to its sophistication.
It is only operational in the US and a few allied countries.
"Nobody has spares of those lying around," Professor Fruehling said.
"And a lot of those systems have now been deployed into Eastern Europe in order to defend those countries should the Russians extend the war."
Polish Defence Minister Mariusz Blaszczak last week said he had asked Germany to send Patriot missile launchers offered to Poland to Ukraine.
Berlin offered Warsaw the Patriot missile defence system to help secure its airspace after a stray missile crashed in Poland earlier in the month.
How long can Russia keep up the barrage?
Russian missile attacks on the Ukrainian capital have already wiped out infrastructure and caused water and electricity outages.
Olga Oleinikova, director of the Ukraine Democracy Initiative at UTS, said Russia is sending a message that, "if it wants to, it can target civilian areas, bombard electricity stations and black out entire cities".
Russia has found a "weak spot" in its air defences and the situation is only going to get worse.
Dr Oleinikova — whose grandparents are still in Kyiv — is worried about the scale of civilian casualties.
"I think it can get quite severe," she said.
"Many of the people who stayed in Kyiv are the old and vulnerable who can't relocate.
"If they can't turn on the lights or heating just imagine how many people are going to die just from the cold weather."
She doesn't believe winter will even pause the fighting and, in the long-term, it will inevitably come down to a dwindling of resources and negotiations.
There are indications that Russia is struggling to keep up with its usage rates.
They have been turning to North Korea and Iran for drone and artillery munitions, and rolling out tanks from storage that were produced in the late 1960s.
Professor Fruehling said the Russians can't keep at this level of intensity, but there was no way to tell when it may slow down.
"How long they can keep up the current usage rates, nobody really knows," he said.
"The Russians will also get to the point where they're going to run out, but we just don't know enough about when that's going to be that you can really plan around that."
