Western superiority in military technology is "eroding", notably at the hands of China and Russia, a leading think-tank said Tuesday in its annual report on the state of militaries around the world.

The International Institute for Strategic Studies (IISS) in London said in its Military Balance report that increasingly easy access to technology by non-state groups means the world faces "an increasingly complex balance of military power".

It also highlighted that Russia and China are pushing to modernise their militaries and are "increasingly active in the development and deployment of advanced military capabilities".

"We previously felt that Western states were the champions of new technology and had a large technological lead either over their state peer competitors or over non-state actors," John Chipman, director-general and chief executive of the IISS, told AFP.

"Now that technological lead is narrowing," he added.

Chipman stressed that military power in the 21st century was not only about planes, tanks and ships but also about technologies such as cyber-security, cyber-weapons and drones.

- Russia as 'growing threat' -

The IISS also said that European defence budgets, which have suffered major cuts since the fall of the Berlin Wall in 1989, were still too low.

Last year, only four out of 26 European members of NATO met a target that two percent of gross domestic product (GDP) should be spent on defence -- Britain, Greece, Poland and Estonia.

The top five spenders on defence last year were the United States on $597.5 billion (528 billion euros), China on $145.8 billion, Saudi Arabia on $81.9 billion, Russia on $65.6 billion and Britain on $56.2 billion.

Russia, which deployed forces to Syria in September, has seen "double-digit" growth in its defence budget, the IISS said.

The United States' overall defence expenditure as a percentage of GDP has fallen from 4.63 percent in 2010 to 3.27 percent in 2015.

However, the amount being spent on the US military presence in Europe is being quadrupled to $3.4 billion.

Chipman attributed this to a "perceived growing threat from Russia, especially to the Baltic states and the eastern European states" and their calls "to reassure them and protect them against Russian intimidation".

In the Middle East, the lifting of sanctions against Iran following a nuclear deal last year raises the possibility of Tehran modernising its military equipment, much of which dates back to the 1970s, according to the IISS.

This could lead to changes in the make-up of countries' armaments in the region.

"Gulf states -- aware that Iran will always have an edge in terms of force size -- will likely look to procure yet more advanced weapons, for instance high-speed precision strike or cruise missiles that would allow them still to reliably engage military targets, even with more capable Iranian opposition," it said.

The nuclear deal struck in Vienna between world powers and Iran has delayed any acquisition by Iran of a nuclear weapon by up to 15 years, Chipman added.