LONDON (Reuters) - A decision by the West to provide arms to Ukraine could prompt a rapid escalation of Russian support for separatists battling government forces in the east of the country, a global security think-tank said on Wednesday.

The International Institute for Strategic Studies (IISS) also said the kind of hybrid warfare it said Russia was conducting in Ukraine represented a “grave threat” to NATO as the military alliance is ill-prepared to deal with it.

U.S. House of Representative members said this week they were preparing legislation to provide arms for Ukraine, something many lawmakers have been pressing for.

This could prompt Moscow, which denies involvement in a conflict that has killed more than 5,000 people, to openly declare it was now supplying arms to the rebels, said Ben Barry, an analyst at IISS and a former senior British army officer.

“Their behavior would be even less constrained and they can introduce additional military support much more quickly than any conceivable external supply of arms to Ukraine,” said Barry, speaking at the launch of the think-tank’s annual assessment of global military capabilities, The Military Balance.

“If the United States and other nations chose to supply Ukraine it would take some time to tip the military balance in their favor and could probably be overmatched by more rapid Russian escalation.”

Key U.S. allies in Europe are much more cautious about providing arms to Ukraine. Leaders of France and Germany joined the presidents of Russia and Ukraine in the Belarussian capital Minsk on Wednesday for a peace summit as fighting flared anew in eastern Ukraine.

IISS welcomed NATO’s ‘spearhead’ rapid reaction force, conceived to reassure member states in eastern Europe, as an important initiative. But it said strengthening military forces on the alliance’s periphery may be more important to deterring future aggression.

IISS said there was “unease” about possible gaps in NATO’s capacity to deal with hybrid warfare, where traditional military tools are combined with non-military such as disinformation, propaganda, provocation and cyber attacks.

“Hybrid warfare represents a grave threat to NATO’s collective security,” it said in the The Military Balance.

“Some current or potential state or non-state adversaries, possibly including ... China and Iran, will learn from Russia’s recent employment of hybrid warfare ... what tactics worked and what capabilities are required to effect results.”