BEIJING (Reuters) - China's export growth probably slowed in August, weighed down by weaker shipments to the U.S. as a temporary boost from Beijing's tariff truce with its top consumer market fades.

The threat of higher U.S. duties on Chinese goods rerouted via other markets and a high base of comparison from last August were also expected to produce a more modest expansion than the previous month.

Outbound shipments were expected to have risen 5.0% in value terms year-on-year, according to the median forecast of 23 economists in a Reuters poll, down from a forecast-beating 7.2% increase in July.

Imports likely grew 3.0%, down from 4.1% a month prior, as a persistent property sector downturn, rising job insecurity, and the tapering of consumer-focused stimulus keep domestic demand in the doldrums. The data will be released on Monday.

U.S. President Donald Trump's erratic trade policy - marked by multiple rounds of tit-for-tat tariff hikes with Beijing and other forms of trade restrictions - has heaped pressure on China's export-oriented economy, posing a serious test to its long-standing growth model.

The world's two largest economies on August 11 agreed to extend their tariff truce for another 90 days, locking in place U.S. levies of 30% on Chinese imports and 10% Chinese duties on U.S. goods, but they appear to be struggling to chart a path beyond the current pause.

Once Trump's tariffs top 35%, they become prohibitively high for Chinese exporters, economists warn.

A visit by senior Chinese trade negotiator Li Chenggang to Washington late last month yielded little of substance.

Meanwhile, China container ship departures for the U.S. continued to fall. They were down an annual 24.9% in the 15 days ending September 3, compared with a 12.4% drop a week earlier, according to data from Citi.

Chinese producers are trying to export more to markets in Asia, Africa and Latin America to offset the impact of Trump's tariffs, but no other country comes even close to U.S. consumption power, which once absorbed over $400 billion of Chinese goods annually.

And with Trump in July threatening a 40% penalty tariff on goods deemed transshipped to the U.S. to evade his earlier levies, Chinese factory owners can no longer find buyers that way. One exporter said the scramble to fill order books elsewhere has descended into a "mad rat race."

China's August trade surplus is forecast to edge up to $99.20 billion, from $98.24 billion in June, but still well below June's $114.7 billion.

Analysts are watching to see whether officials will roll out additional fiscal support in the fourth quarter to spur domestic demand and offset weakening exports.

But policymakers seem to be exercising tighter control over their flagship 'cash-for-clunkers' programme and did not rush replenish funds after several local governments recently ran through the allocation set aside for the scheme.

Last month's acceleration in headline export growth was down to base effects, economists said, a luxury not afforded to the figure due to be released next week, given that China's exports grew at their fastest pace in nearly 1-1/2 years last August.