US-led western powers and Russia have been locked in the worst crisis of the post-Cold War era since Moscow annexed Crimea earlier this year after protesters ousted Kiev's pro-Russian leadership.

More than 300 people have been killed in months-long fighting between Ukrainian troops and separatist insurgents in the country's east over the future of the ex-Soviet state.

Kiev accuses Russia of trying to regain control over its historical domain with the United States expressing alarm over the emergence of fighters from Russia's Chechnya among the rebels.

The United States and its European allies say Moscow is fomenting separatist uprisings in eastern Ukraine, an allegation that Russia has denied.

"We have been reminded that Russia has not gone away as a challenge to Europe and we have to be prepared to defend against that challenge as well as to be active in the Middle East and North Africa," Hammond said in Singapore.

The defence chief told reporters aboard HMS Echo that the situation in Ukraine was "not envisaged" when the United States announced in 2011 that it was rebalancing its forces to the Asia-Pacific.

"I think it's a useful reminder -- while new challenges are emerging, the old challenges haven't necessarily gone away and it has required the Americans in particular to do some nimble footwork to reassure allies in European NATO," Hammond added.

However, Hammond said -- on the sidelines of an Asian security summit in the city-state -- that America's strategic shift to Asia "is the right decision into the future".

"There may be tactical twists and turns in the path but I am sure they will stick to it," he added.

Hammond on Saturday addressed the Shangri-La Dialogue, an annual forum attended by defence chiefs, senior military officers and security officials.