Image copyright Reuters Image caption Mr Trump's campaign spending remains remarkably low to this point

Donald Trump's election campaign doubled its spending last month but still lagged far behind US presidential rival Hillary Clinton, figures show.

The Trump campaign spent $18.5m (£14.1m) in July, compared to his Democratic rival's $38m.

Mr Trump spent more than $420,000 on hats, but only started major TV adverts this month.

Separately, the New York Times said an investigation had revealed Mr Trump's firms were at least $650m in debt.

Mr Trump's campaign spending remains remarkably low at this point compared to previous campaigns, including those of Barack Obama and Mitt Romney in 2012.

The Trump campaign's $18.5m in July was up from $7.8m in June, according to a Federal Election Commission report released on Saturday.

In terms of fundraising, the Trump campaign brought in $37m for the month, compared to the $52m raised by Mrs Clinton.

August will see a spike in Trump spending, however, as last week $5m was spent on the first TV adverts. The Clinton campaign began TV advertising two months ago and has spent more than $60m so far.

Her campaign staff is around 700, about 10 times more than those on Mr Trump's payroll.

Image caption Ledgers show a Trump spending of more than $420,000 on hats in July

The latest figures show a large sum of the Trump July spending, about 45% of the total, went to the web design and digital marketing firm Giles-Parscale, while millions also went on air travel.

They also show Mr Trump is still paying the firm of sacked campaign manager Corey Lewandowski. The company received $20,000.

Mr Trump has financed much of his own campaign - putting in about $50m.

Meanwhile, the New York Times said its investigation had shown that the $650m of debt among Trump companies was twice the figure in public filings made as part of his White House bid.

It also said that some of the lenders backing his ventures, such as the Bank of China and Goldman Sachs, were institutions he had criticised during his campaign.

The Times also said its examination "underscored how much of Mr Trump's business remains shrouded in mystery. He has declined to disclose his tax returns or allow an independent valuation of his assets."

Separately, Mr Trump again reached out to African-American voters, one of the groups he is lagging behind in significantly in opinion polls.

He cited Abraham Lincoln, the first Republican president, who issued the proclamation freeing slaves.

"I want our party to be a home of the African-American voter once again," he said.

Mr Trump's audience in Fredericksburg, Virginia, was overwhelmingly white.