US President Donald Trump on Thursday accused China of backsliding on promises to increase purchases of American farm exports.

The president's latest salvo on Twitter comes the same week that US and Chinese trade officials had their first contact in months in an effort to revive negotiations that nearly collapsed in May.

Trump and his Chinese counterpart Xi Jinping met last month on the sidelines of the Group of 20 summit in Japan, agreeing to cease further hostilities while the talks resumed.

"Mexico is doing great at the Border, but China is letting us down in that they have not been buying the agricultural products from our great Farmers that they said they would," Trump said Thursday on Twitter.

"Hopefully they will start soon!"

Following the Osaka summit, Trump announced that, in return for Washington's pledge to suspend a planned tariff increase on $300 billion in Chinese imports, Beijing had offered to buy "a tremendous amount of food and agriculture product" from the United States.

Hopes for a trade deal with China have lifted spirits on Wall Street but lingering trade conflicts have weighed on US business sentiment and are one of the main dangers on the horizon for the United States, economists say.

Reducing America's soaring trade deficit with China has long been a principal aim in Trump's trade battle with Beijing, which he also accuses of stealing American technology and unfairly intervening in markets.

Since last year, the two countries have traded tariffs on more than $360 billion in two-way trade.

The US global trade deficit has expanded this year but has fallen with China while rising with Mexico, pointing to the realignment of commercial relationships.

Trump has offered billions in aid to buy out US farms badly damaged in the US-China trade war. US soy exports collapsed last year, virtually wiping out foreign markets farmers had spent years cultivating.

Bill Nelson, an analyst at Doane Advisory Services, told AFP the major Chinese purchases that markets had been expecting had so far failed to materialize.

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Chinese soybean purchases zoomed higher in the first quarter of the year to 9.5 million tons, according to official numbers, making up for the sales drought at the end of 2018.

But they have since fallen off sharply, with few major sales recorded since the end of last month.

"China is not making major purchases, certainly not since the Xi meeting," Nelson said.

"Trump implied China would be immediately out there and sales would be going to happen right away. This is not what has occurred."