The balance of power in Syria will eventually shift to rebel forces but a protracted civil war risks destabilising the whole region, a leading think-tank warned Thursday in its annual report on world military strengths.

Syria dominated an "increasingly complex" global security situation that also includes China's rise and a continuing increase in Asian defence spending, the London-based International Institute for Strategic Studies (IISS) said.

Cash-strapped Western militaries are meanwhile using the NATO withdrawal from Afghanistan as a pretext for focusing on smaller but more capable forces, it said.

The "Military Balance 2013" report said the tide was turning against President Bashar al-Assad in the two-year conflict in Syria, even though the prospect of foreign military intervention remained "remote".

"It was likely that, over time, the balance of forces would shift to the rebels, given that their capability and external support would rise," the report said.

"Short of using chemical weapons against rebels, with attendant risk of international intervention, it was difficult to see how Assad could reverse this trend."

But the report warned that regime forces "could still tactically defeat the rebels if the latter abandoned their guerrilla approach and tried to hold urban areas".

"If President Bashar al-Assad could not win, the rebels could still lose," it said.

The rebels also lack strong political and military leadership, the IISS report said.

"This could see the country descend into a civil war with the government just the strongest faction amongst many, increasing the chance of regional destabilisation," it said.

Toby Dodge, the IISS consulting senior fellow for the Middle East, said Assad could struggle on in power for several years.

"The rebels' great problem is that they are not coordinated. Their military forces are fractured, competing, in some cases killing each other," he told AFP.

"They can't launch a coherent, effective assault on the government.

"I cannot imagine a scenario in which the Syrian regime regains legitimacy and coherence but I can imagine it manages to stay in power for the next two or three years."

He said Syria risked becoming a crucible for jihadists, noting that battle-hardened, effective fighters have moved in from neighbouring Iraq and Lebanon.

"That's a profound worry," he said.

"The longer this conflict goes on -- and I think it has got more years to run -- the more Syria will be a magnet for region-wide jihadists and international jihadists."

It could become another Afghanistan, "taken over by radical Islamists who use it as a base from which to strike out into the Middle East and beyond", he warned.

The IISS drew comparisons between the situations in Syria and Afghanistan, saying lessons could be learned because they were "both contests between insurgents and government forces" with external forces involved.

But it said that in Afghanistan, military operations were "overwhelmingly" aimed at minimising casualties, whereas the Syrian government's approach was to use force "as a means and an end in itself and a tool of repression and deterrence."

NATO and the Afghan government are "engaged in a race against the clock" to improve security and build the capacity of the state before the security handover next year, the IISS report said.

Afghan forces are likely to reach full strength by the time they take over from NATO troops at the end of 2014 but the situation after that will likely remain a "patchwork" with continuing insurgent activity.

Globally, a shifting balance was illustrated by the fact that Asia's defence spending overtook that of NATO European states for the first time in 2012, as the Military Balance report predicted last year.

China's rise continued, with the country taking delivery of its first aircraft carrier in September last year and developing a stealth fighter seen as significant.

Austerity has meanwhile prompted many Western militaries to change their focus towards "smaller, though potentially more capable forces" after a decade of wars in Afghanistan and Iraq.

But the IISS report warned: "While doing more with less is a challenge, sometimes numbers count."