Local government administrators in Mali responsible for organising this month's presidential and legislative elections indefinitely extended a strike until demands for better safety and pay are met, a union representing them said on Tuesday.

The administrators' tasks include distributing identification cards needed to vote in the July 29 polls. They first launched a seven-day strike on June 25 to protest against their working and living conditions.

The strike has added to mounting uncertainty about the government's ability to secure the elections. Islamist militants last week raided the headquarters of a regional military base in central Mali, leaving at least six people dead.

Four civilians were also killed on Sunday in a car bomb attack targeting French troops in the north.

Two unions representing the local adminstrators extended the strike on Monday after the government missed a July 1 deadline to meet their demands, Olivier Traore, secretary-general of the National Syndicate of Civil Administrators, told Reuters.

"The strike will impact the organisation of the elections. There was a whole work programme scheduled this week that will now be called into question," Traore said.

But Aboubacar Djire, a technical adviser at the Ministry of Territorial Administration, denied that the strike would affect the elections.

"There are commissions in charge of distributing voting cards, and these commissions continue to work," Djire told Reuters. The government has repeatedly insisted that elections will take place as scheduled.

The distribution of voter cards started on June 20 and is expected to run until July 27.

Mali has been in turmoil since Tuareg rebels and loosely allied jihadists seized its desert north in 2012, prompting French forces to intervene to push them back the following year.

Those groups have since regained a foothold in the north and centre, using the sparsely-populated Sahel as a launchpad for attacks across the region.