BEIJING, Sept 25 (Reuters) - China's exports of rare earth magnets to the United States in August rose to the highest level since at least 2016, customs data showed on Wednesday, pointing to further stockpiling as concerns linger that Beijing could restrict supply in the Sino-U.S. trade war.

Shipments to the United States rose 1.2% from a month earlier to 452,473 kg, or around 452 tonnes, according to data from the General Administration of Customs.

Exports to the United States were 6.2% more than a year earlier and the highest monthly total in customs database records going back to January 2017.

They have been high since Chinese President Xi Jinping visited a rare earths magnet plant in May, with January-August shipments to the United States up 23.8% on year at around 2,984 tonnes.

China is the world's dominant producer of rare earth magnets, which are widely used in medical devices, consumer electronics, and defence. It has raised the prospect of restricting rare earth supply to the United States, although no formal measures have been announced.

Securing the supply of rare earth minerals was on the agenda of a meeting between U.S. President Donald Trump and Australian Prime Minister Scott Morrison in Washington last week, a senior administration official said.

Australia's Lynas Corp is the only major rare earths producer outside of China, whose annual rare earths summit in Baotou, Inner Mongolia, on Sept. 20 was not open to foreign nationals this year.

China's overall rare earth exports, which can fluctuate widely, came in at 4,352 tonnes in August, according to customs data released earlier this month, down 17% from July.

(Reporting by Tom Daly; Editing by Subhranshu Sahu)