A spokeswoman for Russian state conglomerate Rostec confirmed a deal between the two countries had been signed involving Su-35 fighter jets, but declined to provide details.

The deal makes China the first foreign buyer of the Su-35, one of Russia's most advanced military aircraft, and is one of the largest contracts for military jets ever signed between the two countries.

Chinese state media quoted Russian media on the deal. The Chinese government has not commented, in keeping with standard practice on arms deals.

Russia and China have been in talks for several years over the Su-35s, and in 2012 the two sides signed a preliminary agreement for Beijing to buy some of the jets, the Kommersant newspaper reported.

Arms sales are a rare bright spot in an otherwise gloomy economic picture for Russia, whose economy is suffering from weak oil prices and Western sanctions over the Ukraine conflict.

Moscow has sought to deepen trade and financial ties with Beijing following the chill in relations with the West over Ukraine, but some analysts question whether the drive has yielded much in the way of early results.

It is not clear when China will pay for the Su-35s.

A delegation representing China's military and aerospace industry also held talks with Russian state-owned aircraft engine manufacturer United Engine Corp this week on the possible joint development and production of military engines.

This included the AL-41F-1S engine that powers the Su-35, United Engine Corp said in a statement.

While China has developed modern fighter jets like the J-10, J-11 and J-31 in recent years, its engine technology lags behind Russian firms, as well as U.S. and European companies such as United Technologies unit (>> United Technologies Corporation) Pratt & Whitney, General Electric (>> General Electric Company) and Rolls-Royce (>> Rolls-Royce Holding PLC).

Military analysts believe that without advanced engines, China's fighters could be at a disadvantage in combat against aircraft such the U.S.-built Lockheed Martin (>> Lockheed Martin Corporation) F-35 and Boeing (>> Boeing Co) F-15, Europe's Eurofighter Typhoon and France's Dassault (>> DASSAULT AVIATION) Rafale.

Beijing is spending heavily on domestic engine research and development, experts say.

(Reporting by Gleb Stolyarov; Additional reporting by Siva Govindasamy in SINGAPORE and Ho Binh Minh in HANOI; Writing by Alexander Winning; Editing by Andrew Osborn and Dean Yates)

By Gleb Stolyarov