KENTUCKY, USA (Reuters) - US Democratic presidential front-runner Hillary Clinton continued her aim at presumptive Republican nominee Donald Trump on Sunday (May 15), saying the United States should not take a risk on an unreliable candidate.

"I've never heard such reckless, risky talk from somebody about to be a nominee for president than I've heard from Donald Trump when it comes to nuclear weapons," Clinton said at a campaign rally in Louisville, Kentucky.

Clinton, a former secretary of state, has said Trump would have to offer policy specifics in ahead of the November 8 presidential election and criticised him for his positions on issues including nuclear weapons and abortion.

Clinton lost the Democratic primary contest in West Virginia on Tuesday (May 10), but still looks set to win her party's nomination, leading her rival, US Senator Bernie Sanders, by more than 800 delegates.