Oct 29 (Reuters) - Asia's largest oil and gas producer PetroChina Co Ltd 0857.HK reported a 5.3% year-on-year decline in third-quarter net profit as weaker fuel sales and lower oil prices outweighed growing gas and oil output.

Net profit fell to 43.91 billion yuan ($6.15 billion), the company said in a stock filing on Tuesday. Revenue fell 12.4% to 702.4 billion yuan.

PetroChina's net profit remained the highest among domestic peers. Offshore oil and gas producer CNOOC Ltd 0883.HK posted record quarterly earnings of 36.93 billion yuan while top Asian refiner Sinopec Corp's 0386.HK600028.SS halved to 8.5 billion yuan in the same three-month period.

In the first nine months, PetroChina, the listed arm of Asia's top oil producer China National Petroleum Corp (CNPC), produced 708.3 million barrels of oil, 0.3% higher year-on-year, including 585.4 million barrels, or 83%, that were produced domestically.

Natural gas output grew 4% year-on-year to 3,803.8 billion cubic feet during the nine months, 96% of which was produced in China.

Crude oil processing at PetroChina, which is also China's second-largest refiner after Sinopec, was down 0.7% at 1,036.2 million barrels, or 3.78 million barrels per day during the nine-month period.

Meanwhile, the output of chemical products rose 9.7% to 28.64 million metric tons.

That was in line with a 1.6% drop in national refinery throughput in January-September, as the world's second-largest oil consumer faced slowing domestic fuel demand and thin refining margins.

Total sales of gasoline, diesel and jet fuel fell 4.6% year-on-year to 119.9 million tons. Of that, 89.6 million tons were domestic sales, representing a 3.7% fall on the year.

Diesel fuel led the weakness with total sales down 8.6%, and gasoline sales fell 4.6%. Aviation fuel expanded 12.6% thanks to recovery in China's outbound travels.

Capital spending totalled 186.9 billion yuan in the first three quarters, 7.4% higher versus a year earlier.

PetroChina's Hong Kong-listed shares have risen 12.6% year-to-date, lagging a 21.4% rise in the Hang Seng index .HSI.

($1 = 7.1377 Chinese yuan renminbi)

(Reporting by Colleen Howe in Beijing and Chen Aizhu in Singapore; Editing by Sharon Singleton)

((colleen.howe@thomsonreuters.com;))