* Aluminium exports slip 3.6 pct from Sept to lowest since May
Domestic output has been falling amid low aluminium prices
* New U.S. tariffs will start to impact exports -analyst
* Steel shipments drop to lowest since Feb
BEIJING/MANILA, Nov 8 (Reuters) - China's aluminium exports fell in October as sliding domestic production meant less metal was available for overseas markets, while steel exports also dropped, hitting their weakest since February amid stiffer competition in key market Southeast Asia.
China's unwrought aluminium and aluminium product exports came in at 482,000 tonnes last month, the General Administration of Customs said on Thursday, down 3.6 percent from a revised figure of 500,000 tonnes in September but still up 37.7 percent from October 2017.
The October aluminium export volume, in a month that included a week-long holiday around China's National Day, is the lowest since May, with shipments having reached at least 500,000 tonnes in the four months from June to September.
The United States in March imposed import tariffs of 25 percent on steel and 10 percent on aluminium, including from China, in one of the opening moves of an ongoing trade dispute between the world's top two economies.
Several steel and aluminium products from China were hit with additional 25 percent tariffs by the United States in September.
"October was one-week short, so it's a pretty good number on a work day basis," said Paul Adkins, managing director of aluminium consultancy AZ China.
"We expect exports to start slipping a bit more" after the additional tariffs imposed in September, he added, noting that transshipments of Chinese metal to the United States via third countries should be watched.
Washington on Wednesday imposed final anti-dumping and anti-subsidy duties of 96.3 percent to 176.2 percent on Chinese common alloy aluminium sheet products.
Exports from China, the world's No.1 producer of steel and aluminium, have benefited recently from a weakening yuan, while shipments of semi-fabricated aluminium have just received an added incentive in the shape of a higher export tax rebate effective Nov. 1.
Chinese steel product exports came in at 5.5 million tonnes in October, customs said, the lowest since February. Last month's total was down from 5.95 million tonnes in September but up 10.4 percent from a year earlier.
Strong competition in the Southeast Asian market, a key destination for Chinese steel exports, curbed shipments, said Richard Lu, analyst at CRU consultancy in Beijing.
"Competitors like Turkey, Middle East, Russia - all of them can supply much cheaper products than the Chinese and China's prices are very high given the strong domestic market," he said.
Some Chinese exporters have deliberately reduced volumes for export to sell more at home, said Lu.
For more details, click on TRADE/CN (Reporting by Tom Daly and Manolo Serapio Jr. Editing by Joseph Radford)