BRUSSELS (AP) — World leaders on Wednesday waited to see if Russian President Vladimir Putin would cast the die and order troops deeper into Ukraine. At the same time, they worked to maintain a united stance and vowed to impose tougher sanctions in the event of a full-fledged invasion.

Fears of an imminent offensive were further heightened late Wednesday when the Kremlin said rebel leaders in eastern Ukraine have asked Russia for military assistance to help fend off Ukrainian "aggression."

With Russian lawmakers having authorized Putin to use military force outside the country and Ukraine surr...