UPDATE 1-China extends Rio probe by two months - Australia

(Adds more quotes from Smith)

By Dean Yates

SINGAPORE, Nov 12 (Reuters) - China has extended by two months a commercial espionage investigation into a Rio Tinto executive, Australia's foreign minister said on Thursday, in comments that suggest no early resolution to a case that has strained ties.

Stern Hu, an Australian, was formally arrested in August along with three Chinese executives from the Anglo-Australian miner.

"The Chinese authorities, in accordance with Chinese law, have extended the investigation period for a further two months so that will take the investigation period, as I understand it, to mid-January," Foreign Minister Stephen Smith told reporters on the sidelines of an Asia-Pacific meeting in Singapore.

Chinese authorities had previously extended the investigation for one month in October. Smith did not comment on the other three executives.

China is Australia's biggest trading partner. Two-way trade was worth A$76 billion ($70 billion) in the year to June 30, with China buying more than A$25 billion worth of Australian iron ore and coal.

Smith said he would raise Hu's case with his Chinese counterpart, Yang Jiechi, at the Asia Pacific Economic Cooperation forum meeting in Singapore later on Thursday.

"We continue to say to the Chinese authorities, as I will to Foreign Minister Yang, that whilst we understand this matter is being progressed in accordance with Chinese law we do think it is in everyone's interests for the matter to be expedited and resolved as quickly as possible."