HANOI, April 24 (Reuters) - Vietnam is in talks with the Czech Republic for military supplies, including aircraft, radars, upgrades of armoured vehicles and firearms, a Czech government source told Reuters, as Hanoi aims to diversify its mostly Russian arsenal.

Security was one of the main topics discussed with Vietnam leaders by Czech Prime Minister Petr Fiala in a three-day visit last week, the official said, noting that security firms were the largest component of the business delegation that accompanied Fiala.

The former Soviet satellite state is seen as being well positioned to meet some of Vietnam's growing security needs as its military firms excel in retrofitting Russian gear and often manufacture new equipment that is compatible with Soviet legacy arms - a skill that is particularly well regarded in Vietnam, where 80% of the local arsenal is estimated to come from Russia.

Over the last two decades, Prague has already positioned itself as the European Union's main supplier of weapons to Vietnam, according to data from the Stockholm think-tank SIPRI.

Hanoi ordered a dozen L-39NG light-combat aircraft in 2021 from Czech manufacturer Aero Vodochody, with delivery to begin this year. There are talks to deliver more such planes, said the source, who attended the high-level meetings and asked not to be named because the talks were internal.

At the end of a meeting between Fiala and Vietnamese Prime Minister Pham Minh Chinh, the Vietnamese government said "the two leaders agreed to step up cooperation in security-defence", among other fields.

The official said that Prague could support the transfer of weapons technology and local manufacturing if significant supply deals were concluded.

Of the 15 companies that made up the Czech business delegation, four were security firms. They were Czechoslovak Group, Colt CZ Group, Omnipol and STV GROUP, the official said.

The companies did not reply to requests for comment and Colt declined to comment.

Omnipol has a minority stake in Aero Vodochody and owns Aircraft Industries, the Czech maker of L 410 NG cargo aircraft, whose sale was also discussed last week with Vietnamese military procurement officers, the official said.

Similar talks were held in past days with Indonesian civilian officials and Philippines counterparts, as part of the Czech delegation's tour of Asian countries.

Omnipol officials also held talks in Vietnam about the possible sale of dual-use radar to be installed in civilian and military airports, the official said.

In Hanoi, STV Group and Czechoslovak Group discussed possible contracts to upgrade Vietnam's Soviet-made tanks and armoured vehicles with advanced technologies including communication equipment.

Under those contracts, the companies could also provide spare parts and maintenance, the source said, noting that talks were still preliminary and no new deal was signed last week.

"You don't have many options if you want to keep the old Soviet equipment working. And possibly to do it in a more sophisticated way," Czech Ambassador to Hanoi Hynek Kmonicek told Reuters.

An executive at Excalibur Army, which is part of the Czechoslovak Group, said talks on possible deliveries of new armoured vehicles, rocket launchers and howitzers were progressing very slowly.

The Vietnamese foreign ministry did not reply to a request for comment.

Possible sales of firearms were also discussed with Colt CZ Group, the Czech holding company that owns the long-established U.S. carbine and rifle maker, the source said. (Reporting by Francesco Guarascio @fraguarascio; additional reporting by Khanh Vu; Editing by Raju Gopalakrishnan and Kevin Liffey)