U.S. and Russian defence firms on Thursday displayed weapons and promoted models of aircraft at Vietnam's first large-scale arms fair, as the two powers vie for influence and arms sales in the strategic Southeast Asian country that borders China.

The event at an airbase in the capital Hanoi attracted 174 exhibitors from 30 countries, including all large arms-making nations except China, as Vietnam signalled its interest in diversifying military procurements and possibly starting exports of its own weapons.

The arms fair "represents a new stage in Vietnam's efforts to globalise, diversify and modernise, and the United States want to be part of it," U.S. ambassador in Vietnam Marc Knapper told media on the sidelines of the event.

He said the United States wanted to boost its military cooperation with Vietnam - limited mostly to coastguard ships and trainer aircraft - and was ready to discuss its defence needs, especially on maritime capabilities.

Vietnam and China have been locked in a long-running territorial dispute over archipelagos and energy exploration in the South China Sea, with concerns it could one day lead to confrontation.

China was invited to join the arms fair but declined the offer.

Vietnam has had the option to buy U.S. weapons only in the past few years following Washington's gradual removal between 2014 and 2016 of an arms embargo on Vietnam, which was one of the last vestiges of the Vietnam War.

Hanoi aims to diversify arms sources "for the purpose of protecting the nation and the people amid rising conventional and unconventional challenges," Prime Minister Pham Minh Chinh said at the event's opening.

Diversification is widely seen as a byword for reducing dependence on Russia for weapons, although analysts underline that any possible shift would be gradual.

Russia's arms trade agency Rosoboronexport was present at the fair with a large booth displaying Russian drones, armoured vehicles, helicopters, planes and small arms.

The agency "is ready to discuss cooperation in the field of industrial partnership and the construction of infrastructure facilities," Director General Alexander Mikheev said in a statement.

Russia is by far the main weapons supplier to Vietnam, covering 80% of its needs, but its appeal has decreased recently, while the Ukraine war could constrain its defence exports and sanctions deter potential buyers. (Reporting by Khanh Vu and Francesco Guarascio Editing by Ed Davies, Martin Petty)