U.S. spinning mills have increased their sales of yarn to China almost fivefold this year, taking advantage of voracious demand from the world's largest textile market where clothes makers are struggling to source low-priced cotton.

Spying an opportunity to boost profits after being roiled by historically high prices for cotton and a sluggish domestic retail market, U.S. mills, including Frontier Spinning Mills, one of the country's largest, have ramped up sales to China, mills said.

Mills in China have been buying more cotton yarn, a semi-finished thread with lower import duty rates, since last year in a bid to bypass duties on the raw fiber. Chinese mills have bought their yarn mainly from Bangladesh, Turkey, Pakistan and Southeast Asia.

The move is a dramatic shift in textile trade for the United States. The country is the world's largest exporter of raw cotton, but annual shipments of 400,000 tonnes of yarn of 85-percent or higher cotton content only account for a small portion of global yarn trade. Most of that heads to South America.

Reduced energy costs and sophisticated technology have helped to restore some of the U.S. textile industry's competitive power which it lost to countries with lower labor costs. U.S. yarn is less labor intensive because of high-tech machinery, market participants said.

The trend is also significant for the global market, illustrating how China's aggressive stockpiling policy has transformed trade flows and boosted prices of critical raw material.

But the much-needed boost for the U.S. textile industry may be short-lived as Beijing prepares to overhaul its buying program, which will likely ease supply constraints on China's mills and reduce their need to buy yarn.

The pace of China's buying has been remarkable by any measure.

The country leapfrogged Mexico and El Salvador to third place behind Honduras and the Dominican Republic last year as a top export destination even as total exports shrank by a sixth, U.S. International Trade Commission data shows.

Honduras and the Dominican Republic still take the lion's share of U.S. cotton yarn because of existing infrastructure and the Central America Free Trade Agreement.

While the shift in yarn shipments started at the tail end of 2012, it has taken off this year. China bought as much of the semi-processed product in the first four months of the year as it did for the whole of 2011 and of 2012.

U.S. exports of yarn to China totaled 22,300 tonnes in the first four months of the year, up from 4,800 during the same period in 2012, the ITC reported.

That is also more than the 21,300 tonnes China's mills bought in all of 2012 and the 16,500 tonnes they purchased throughout 2011.

China is now the only destination for yarn shipments in the top 10 locations outside of North and South America.

"It's been happening in small amounts for a while, but it's been increasing. It's a pretty novel change, it's pretty dynamic," said one European cotton trader.

The move has paid off. Total U.S. exports in the first four months totaled more than 137,600 tonnes, some 15 percent ahead of the same period last year. Even so, few believe it will last for long.