Europe has increased defense spending since Russia invaded Ukraine, but its defense capacities, including military workforce, are still insufficient, a British security think-tank warned on Friday.

The findings by the International Institute for Security Studies (IISS) come as Donald Trump's return to the White House has raised fears that he could upend European security and pull the plug on support for war-torn Ukraine.

The IISS published its latest report as it hosts the Prague Defense Summit, bringing together politicians, army officials and other experts to discuss ways to boost Europe's defense posture.

Russia's invasion of Ukraine in February 2022 uncovered multiple weaknesses in Europe's ability to defend itself, the IISS study said.

"Defense spending of NATO's European members in 2024 is almost 50% higher than it was in 2014 when Russia annexed Crimea," it said.

Nonetheless, European armed forces "continue to rely on the United States to varying degrees across all military domains," the report warned.

Europe's arsenal was "severely depleted due to political decisions after the end of the Cold War and the following decades. In the process, the defense industry in Europe also contracted," the study said.

However, production in some sectors, including air defense and artillery, has increased markedly since 2022 as arms makers respond to Ukraine's needs.

"For instance, Rheinmetall's global annual 155 mm ammunition production rate has increased tenfold to 700,000," the study said, referring to Germany's arms giant.

In recent years, European countries also have been buying more weaponry from local producers as the continent's NATO members have used more than half of their spending on European systems since February 2022, against four spent on U.S. systems.

While the production of weapons and ammunition is growing, countries across Europe are grappling with a lack of military personnel, the IISS said.

"Key European armed forces remain under-strength ... too many continue to lose troops while not incentivizing enough of the younger generation to volunteer," it said.