WASHINGTON (Reuters) - U.S. President Donald Trump on Tuesday vowed to protect American farmers against China by signaling to provide further aid if needed, a day after Chinese firms stopped agricultural purchases and Beijing threatened more tariffs on U.S. farm products.

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“Our great American Farmers know that China will not be able to hurt them in that their President has stood with them and done what no other president would do - And I’ll do it again next year if necessary!” Trump tweeted on Tuesday.

U.S. farmers, a key political constituency for Trump, have been among the hardest hit in the trade war between the world’s two largest economies. Shipments of soybeans, the most valuable U.S. farm export, to top buyer China sank to a 16-year low in 2018.

To compensate for the losses, the Trump administration has rolled out up to $28 billion in federal aid since the trade war began last year, and the Agriculture Department to date has made $8.6 billion of direct aid payments to farmers.

The latest federal aid package of up to $16 billion was rolled out in July and the first tranche for the payments are expected to begin in mid-to-late August.

For a graphic on Trump's trade war farmers bailout, click tmsnrt.rs/2YoyTpk

Promises of large agricultural purchases by China have been a key element of a potential trade deal, but Trump last week said Beijing had not fulfilled a promise to buy large volumes of U.S. farm products and vowed to impose a 10% tariff on a further $300 billion in Chinese imports starting Sept. 1.

On Monday, China’s Commerce Ministry said Chinese companies have stooped buying U.S. farm products and added that Beijing may also impose additional tariffs on American agricultural goods, a move that could further hurt U.S. farm states that are key for Trump’s 2020 re-election bid.

Overall, China has purchased about 14.3 million tonnes of last season’s soybean crop, the least in 11 years, and some 3.7 million tonnes still need to be shipped, according to U.S. data. China bought 32.9 million tonnes of U.S. soybeans in 2017.

National Economic Council Director Larry Kudlow also said the administration would help farmers more if need be. “The farmers have been great, they’re patriots. They backed us 100%,” he told reporters. “But sure, we’re helping them as much as we can. We will help them more if need be and we’ll see how the negotiations go.”