LONDON: Drones will become an increasingly common tool of warfare and surveillance as their cost falls, a leading think-tank said Wednesday, although humans will retain ultimate control over lethal strikes.

The International Institute for Strategic Studies (IISS) also noted a continuing trend of Asian military spending surging ahead as European defence budgets shrink, in its annual assessment of the world's armies.

At the launch of the Military Balance 2014 at the IISS's London headquarters, its military aerospace expert Doug Barrie said the use of unmanned aerial vehicles (UAVs), or drones, would increase, although they would continue to co-exist next to manned aircraft.

He said the assumption entering the 21st century had been that autonomous drones would soon completely replace piloted planes.

“I think there's been a step back from that to some extent. I think you will see mixed use for quite some time,” Barrie said, but acknowledged that the range of drones' capabilities would increase.

“We're going to see more of these things. They will proliferate from the kind of system you can stick in your backpack up to full-blown combat strike,”he added.

'More drones means more ethical questions'

The IISS said the increased use of drones was accompanied by legal and ethical questions, including whether attacks could be justified as self-defence and whether they constitute a proportional response to the status of individuals targeted.

There has been particular concern over the potential use of fully autonomous armed UAVs, without humans piloting the devices from the ground.

“Machine-based decision-making as the basis for lethal action will remain a threshold legislatures and the public will likely be unwilling to cross,” the report said.

The report said that drones were once seen almost exclusively in Western armed forces, but the proliferation of smaller systems had reduced costs, enabling greater use by private companies, individuals and countries with limited financial resources.

UAVs have been used overwhelmingly in intelligence, surveillance and reconnaissance, in effectively uncontested airspace, the IISS noted.

However, such calculations may change if they were up against active air defence weapons. Russia and China have been developing GPS-jamming technology, the think-tank said.

In ongoing trends, the report noted the relative shift towards Asia in the balance of military power, with defence budgets in the region rising as Western ones tighten.

The United States remained by far the world's biggest defence spender in 2013, with a budget of $600.4 billion, the report said, followed by China ($112.2 billion), Russia ($68.2 billion) and Saudi Arabia ($59.6 billion).

In terms of defence budgets as a share of gross domestic product, Afghanistan was top on 13.8 per cent, followed by Oman (11.7 per cent), Saudi Arabia (eight per cent) and Iraq (7.2 per cent).

But in real terms, Asian defence spending was 11.6 per cent higher in 2013 than in 2010, while it had fallen by 2.5 per cent in Europe.

'Nuclear suicide mission'

Experts said maritime drones were lagging behind UAVs but there was now substantial investment.

Meanwhile, “we've seen an explosion of unmanned systems on land, with armies having lots of drones, and with small drones, the genie's out of the bottle,” said IISS land warfare expert Ben Barry.

In Syria, Barry said neither side had a clear advantage, nearly three years into the civil war.

“The rebels are probably hampered as much by the lack of unity between the various groups as the factional infighting,” he said.

“I think it's unlikely that we'll see a decisive shift on the battlefield for at least six months.”

For North Korea, while its ageing aircraft could not penetrate South Korean or Japanese air defences with a nuclear weapon, “a nuclear suicide mission by a mini-submarine cannot be ruled out”.

As Western forces withdraw from Afghanistan this year, Nato faces pressing issues at its September summit in Britain, working out what shape the “post-operational alliance” should take.