Vietnamese activists chanted anti-China slogans in Hanoi Wednesday as they marked the 37th anniversary of a border war with their giant neighbour, in a memorial that followed reports that Beijing has installed missile systems in contested seas.

The two communist countries are locked in a long-standing territorial dispute over the Paracel and Spratly islands in the South China Sea.

Vietnam's premier is due to return home Wednesday after attending a summit hosted by the United States aimed at bolstering regional resolve in the face of China's military muscle-flexing.

On Wednesday images relayed by Fox news appeared to show two surface-to-air missile systems installed by Beijing on Woody Island in the Paracels chain, which is also claimed by Taiwan.

In a pre-planned event, more than 100 people gathered in Hanoi to mark an older territorial conflict: China's 1979 invasion of Vietnam's northernmost provinces.

Security officials stood by as veterans chanted "down with China, down with China's invasion."

The short but bloody war came after Vietnam toppled the Beijing-backed Khmer Rouge regime in neighbouring Cambodia.

It claimed tens of thousands of lives and ended with Chinese forces withdrawing but both powers claiming victory.

Although Vietnam fetes its military victories over the French and American armies, it has not arranged any official events to mark the China border war -- much to the chagrin of veterans and activists.

"We are very sad that there has never been any official organisation (memorial) for the day," 64-year-old Vietnamese war veteran Pham Thanh told AFP at Wednesday's commemoration in central Hanoi, where demonstrators carried banners that read "we will never forget".

Beijing's increasingly assertive stance in contested waters has triggered public anger and rounds of protests in authoritarian Vietnam where the demonstrations are sometimes forcefully broken up.

China is also Vietnam's largest trading partner, complicating Hanoi's position towards its neighbour.

On Tuesday, US President Barack Obama announced plans to make his first state visit to Vietnam in May, which Vietnam's foreign ministry hailed as "carrying Vietnam-US relations to new heights".