Footage shared online showed several helicopters flying low in formation over the Lumut naval base, located about 160 kilometres (100 miles) from the capital Kuala Lumpur.

The helicopters were participating in a flypast rehearsal ahead of Naval Day celebrations in May.

One chopper was seen clipping the rear rotor of another, causing both to go into a tailspin and crash.

"The two helicopters collided during flight training," said Suhaimy Mohamad Suhail, senior operations commander from the fire and rescue department, adding that all 10 crew members on board were confirmed dead by medical officers.

The two aircraft involved were a Eurocopter AS555SN Fennec and an AgustaWestland AW139, the rescue agency said.

Photos showed the Eurocopter model heavily mangled in the wreckage on the naval base stadium track with rescue personnel surrounding it as well as various debris.

The AgustaWestland helicopter crashed at the naval base's swimming pool area.

- 'National heroes' -

Prime Minister Anwar Ibrahim sent his condolences to the families of the victims, saying "the nation mourns the heart-wrenching and soul-wrenching tragedy".

"I was informed that an immediate investigation will be carried out by the Ministry of Defense, especially TLDM (Royal Malaysian Navy), to find the cause of the crash," he said.

Malaysia's King Sultan Ibrahim Sultan Iskandar also shared condolences.

"Queen Zarith and I feel very sad over the loss of national heroes in this tragedy," he said in a social media post.

Helicopter crashes are not uncommon in the Southeast Asian nation.

Last month, all four people onboard a Malaysian coast guard helicopter were rescued after it crashed in the Strait of Malacca.

The helicopter was on a training flight when it plunged into the waterway, a narrow shipping lane which runs between Malaysia and the Indonesian island of Sumatra.

Two people were killed in 2020 when a pair of helicopters from the same Malaysian pilot school collided during a training flight, according to authorities.

In 2016, a deputy minister was among those who died after a Eurocopter AS350 crashed in the Malaysian state of Sarawak.