Vietnam is in talks with European and U.S. contractors to buy fighter jets, maritime patrol planes and unarmed drones, sources said, as it looks to beef up its aerial defenses in the face of China's growing assertiveness in disputed waters.

The battle-hardened country has already taken possession of three Russian-built Kilo-attack submarines and has three more on order as part of a $2.6 billion deal agreed in 2009. Upgrading its air force will give Vietnam one of the most potent militaries in Southeast Asia.

The previously unreported aircraft discussions have involved Swedish defense contractor Saab, European consortium Eurofighter, the defense wing of Airbus Group, and U.S. firms Lockheed Martin Corp. and Boeing, said industry sources with direct knowledge of the talks.

One Western defense contractor said Hanoi wants to modernize its air force by replacing more than 100 aging Russian MiG-21 fighters while reducing its reliance on Moscow for weapons for its roughly 480,000-strong military.

Vietnam has ordered about a dozen more Russian Sukhoi Su-30 front-line fighters to supplement a fleet of older Su-27s and Su-30s.

U.S. Defense Secretary Ash Carter, during a visit to Vietnam on Sunday, pledged $18 million to help Hanoi buy U.S. patrol boats.

But any deal with Lockheed or Boeing will likely be the most significant involving a U.S. firm since Washington started easing a longtime embargo on the sale of lethal weapons to Vietnam in October.