Ukraine has renewed its call for more weapons supplies to resist Russia's invasion.
President Volodomyr Zelensky told this week's meeting of Nato: "We need to break the Russian artillery advantage. We need much more modern systems."
If Ukraine did not receive the weapons it needed to defeat Russia, he said, Nato leaders would face a future war with Moscow themselves.
Huge amounts of military equipment have been given to Ukraine by more than 30 countries, but heavy weaponry has been slow to arrive and in some areas Ukrainian troops have been heavily out-gunned.
Which countries are giving the most?
In terms of spending, the US has committed to providing far more than any other individual country.
The UK and Poland have committed to spending the second and third largest amounts.
In terms of money already spent, rather than commitments to spend, the White House says the US has provided $6.3bn (£5.2bn) in security assistance to Ukraine since President Joe Biden took office in January 2021.
President Zelensky has appealed for more funding, and has said the monthly cost of defence for Ukraine was about $5bn (£4.1bn).
What are the key systems?
Military professionals say success on the battlefield requires a huge range of weaponry, as well as training, spare parts and other support.
"No weapon system is a silver bullet," according to General Mark Milley, US Chairman of the Joint Chiefs of Staff.
However, several weapon systems are believed to have played key roles in the conflict so far.
With Ukrainian positions in the east of the country under intense Russian bombardment, analysts say Ukraine badly needs better supplies of artillery and ammunition to hold on to key positions.
So far, it is thought that 10 long-range multiple rocket launchers have been either delivered to Ukraine or are on the way, from the US, UK and Germany.
Ukraine says many more are needed to stall Russia's advance.
The US systems are M142 High Mobility Artillery Rocket System or Himars.
Crucially, the range of Himars, and many other systems, varies according to the munitions used, and it is believed that western donors have not provided the ammunition with the longest range.
The munitions thought to have been supplied to Ukraine give the system a range of about 50 miles (80km), further than the Smerch system on the Russian side.
Himars is also much more accurate than the equivalent Russian systems.
Australia, Canada and the US have also sent more than 100 M777 howitzers and 300,000 rounds of 155mm ammunition to Ukraine.
The range of the M777 is similar to Russia's Giatsint-B howitzer, and much longer than Russia's D-30 towed gun.
Ukraine's own Warsaw Pact-designed artillery uses 152mm shells.
But with stocks running low, Ukraine is shifting to the Nato-configured 155mm ammunition.
Re-orienting Ukraine's ammunition supplies is complicated and difficult, and reports suggest that Ukrainian forces are suffering a serious shortage in some areas.
The West has been relatively slow to respond to Ukraine's request for heavy weapons.
In the early stages of the war there were concerns about provoking Russia.
Politicians also seriously underestimated the level of Ukrainian resistance..
Over time those attitudes have changed, though there are still questions about the west's resolve to keep supplying Ukraine with arms.
At first the focus was on providing Ukraine with compatible weapons - the ones they've been trained to use.
That included soviet era tanks, air defence systems and ammunition.
The US and other allies helped scour Europe for similar weapons, but those stockpiles have been slowly running dry.
So there is now a transition to sending more modern western weapons with the hope that it will be more sustainable in the longer term.
But that has brought more challenges. Modern weapons systems are often more complex and require training, not just in how to operate them, but to maintain and repair them.
Western weapons are also designed for a Western way of war, one that's focussed more on precision than quantity and mass.
For Ukraine, fighting for its life against overwhelming odd, numbers still matter. Ukraine still believes it's not getting enough weapons.
At the very start of the war Ukraine appealed for more fighter jets. So far not one has been delivered.
At least 5,000 shoulder-launched Nlaw weapons, designed to destroy tanks with a single shot, have been supplied to Ukraine.
The weapons are thought to have been particularly important in stopping the advance of Russian forces on Kyiv in the hours and days following the invasion.
"Nlaw was absolutely critical to the defeat of Russian ground thrusts in the early stages of the war," according to Justin Bronk of the Royal United Services Institute.
Ukraine has received more than 230 Warsaw Pact-designed tanks from Poland and the Czech Republic.
Ukraine's armed forces have been using T-72s for decades and have maintenance and spare parts capabilities, in addition to trained crew.
Poland's donation of tanks has been partly back-filled by alternative weaponry from allied nations, including Challenger 2 tanks from the UK.
Drones have featured heavily in the conflict so far, with many used for surveillance, targeting and heavy lift operations.
Turkey has sold Bayraktar TB2 armed drones to Ukraine in recent months, whilst the Turkish manufacturer of the system has donated drones to crowd-funding operations in support of Ukraine.
Analysts say the Bayraktar TB2s have been extremely effective, flying at about 25,000 feet (7,600m) before descending to attack Russian targets with laser-guided bombs.
They are believed to have destroyed helicopters, naval vessels and missile systems.
They have also been used to provide the exact locations of Russian positions for precision artillery strikes.
Ukraine has successfully denied Russia full control of Ukrainian airspace during the conflict, but has called repeatedly for better air defence systems.
In the coming days, Washington is expected to announce that it will send NASAMS, an advanced surface-to-air missile system to Ukraine.
Kyiv has also received S-300 air defences from Slovakia.
Graphics by Gerry Fletcher and Sana Dionysiou