An international network of MH370 next-of-kin said Monday it was "gravely concerned" that the search for the missing jet could end by August, calling instead for it to be extended and expanded.

The grouping, known as Voice370, appealed to countries involved in the expensive operation, aircraft manufacturer Boeing and the International Civil Aviation Organisation (ICAO) to commit resources to continue the search in the southern Indian Ocean.

"We are gravely concerned about the impending completion of the search in the current targeted area," the group said in a statement to AFP.

"It has always been, and still remains, in everyone's best interest that this plane is found."

Australian Transport Safety Bureau chief Martin Dolan, who is in charge of the search, said last week that deep-sea sonar scanning of a designated 120,000-sq-km (46,332-sq-mile) area was likely to finish by early August.

In comments to The Australian newspaper, he added there was no indication the search zone would be enlarged.

The Malaysia Airlines Boeing 777 vanished on March 8, 2014 en route from Kuala Lumpur to Beijing, with 239 people -- including 152 from China -- aboard.

The cause of the disappearance of Flight MH370 is still unknown and it remains one of the greatest mysteries in aviation history.

Australia, Malaysia and China have said their joint search would likely be abandoned if nothing is found in the current designated zone.

Many family members still suspect a cover-up in the mystery and have repeatedly called for the most expensive search operation in history to continue until something is found.

Authorities have said the cost of the search could be up to $130 million.

Five pieces of debris which have been identified as either definitely or probably from the jet have been discovered thousands of kilometres from the search zone -- in South Africa, Mauritius, Mozambique, Mauritius and the island of Reunion -- likely swept there by currents.

Voice370 said it was "appalled" at the lack of a concerted international effort to round up more such debris and piece together what happened.

"A highly concerted effort must be made to find more pieces if the authorities are interested to solve the mystery," it said.

"The biggest, most unprecedented aviation mystery of all time warrants unprecedented action."