Democratic presidential candidate Hillary Clinton has declared victory in Kentucky as she tightens her grip on the presidential nomination.

With most of the votes counted in the state and less than 0.5% ahead, Clinton was declared the unofficial winner by one state official.

In the night's other primary contest Senator Bernie Sanders won Oregon.

Front-runner Clinton is almost certain to secure the nomination in July, with a significant delegate lead.

Alison Lundergan Grimes, chairwoman of the Kentucky State Board of Elections, told CNN that unofficial results confirmed that Clinton would narrowly win the state's primary contest.

Shortly afterwards, Clinton tweeted: "We just won Kentucky! Thanks to everyone who turned out. We're always stronger united."

In the Republican race, Donald Trump won the party's only contest on Tuesday, in Oregon, which was no surprise as he was the last candidate in the race.

The Kentucky Democratic primary will award 60 delegates to go to the party's convention in Philadelphia while Oregon's primary will award 74.

Pressure is rising on Sanders, a senator from Vermont who has historically been an independent, not a Democrat, to drop out of the race.

Some Democrats worry that his presence is hurting their chances of beating Trump, a billionaire businessman with no political experience, in the general election in the autumn.

Sanders has argued that he still has a path to the Democratic nomination.