BRISBANE, Australia—Russia and Ukraine loomed over a weekend meeting of the Group of 20 major economies, with swipes at Moscow by the U.K. setting the stage for a possible confrontation with President Vladimir Putin.

As world leaders arrived here for the summit, British Prime Minister David Cameron and his Australian counterpart, Tony Abbott, met in the capital, Canberra, where the British leader threatened tougher sanctions against Moscow amid unconfirmed reports of Russian troops and tanks entering Ukraine.

...