0412 GMT April 26, 2019
Obama made the statements on Tuesday just a day after lifting a decades-long ban on the sale of lethal arms to Vietnam, a move which is likely to anger China and pave the way for an escalation of tensions in the region.
"In the South China Sea, the US is not a claimant in current disputes, but we will stand with our partners in upholding key principles like freedom of navigation," Obama said.
Obama said Washington supports Vietnam’s territorial claims against Beijing in the South China Sea and promised it greater access to military equipment.
"Vietnam will have greater access to the equipment you need to improve your security," Obama said. "Nations are sovereign and no matter how large or small a nation may be its territory should be respected."
In a wide-ranging speech, the US president also sought to promote the Trans-Pacific Partnership (TPP), a US-led trade deal that has attracted considerable criticism in recent months.
Obama arrived in Hanoi on Sunday for three-day visit. He is the third sitting US president to visit the country since the end of the Vietnam war.
The US carried out an eight-year military intervention in the country from 1965-73.
The two countries have improved ties over the past two decades, with annual bilateral trade now standing at about $45 billion.
While Vietnam wants to work with the US to challenge China’s expansive territorial claims on the South China Sea, it is concerned about irritating its powerful neighbor, a fellow Communist-run state with which Vietnam shares a complex set of security, trade and political ties.
Hanoi has a complicated past with Beijing, which controlled much of northern Vietnam for centuries.
Across Southeast Asia, concerns about China and its growing military have created an opportunity for the US to improve relationships.
In recent years, American aircraft and ships have returned to the Philippines for the first time in more than two decades, while US Marines have started training in Australia and new guidelines have allowed for closer cooperation with Japan.
China claims nearly all of the South China Sea, despite partial counterclaims by Brunei, Malaysia, Taiwan, Vietnam and the Philippines. China is also locked in disputes with Japan and South Korea over the East China Sea.