* U.N. mission replaces African force, will double in size
* Doubts linger over election due on July 28
* Mali seen as model democracy before 2012 implosion
The United Nations took over command of an African peacekeeping force in Mali on Monday, bolstering the mission in a country still threatened by militants and weeks away from what analysts warn could be chaotic elections.
The transfer of command will bring in soldiers from beyond Africa and eventually see the operation more than double in size.
Western and regional powers want to keep order in the West African country after a tumultuous 18 months when soldiers toppled the president and al Qaeda-linked rebels seized the desert north.
France, fearing the militants could use the territory as a launchpad for foreign attacks, launched a lightening offensive in January and forced them to retreat.
Paris still has troops in the country but wants to hand over most security responsibilities to the United Nations.
The U.N. mission, known as MINUSMA, will be the world body's third largest with 12,600 soldiers and policemen once fully deployed.
"MINUSMA's military force will be reinforced gradually in the coming months," mission chief Bert Koenders said at the launch ceremony in Bamako.
"Contingents will deploy in the main population centres in northern Mali ... But MINUSMA cannot do everything. We are here to support the efforts of the government and its partners."
The U.N. force will operate alongside troops from former colonial power France, some of whom will remain in the country to tackle remaining Islamists.
There are currently around 6,000 troops from mainly West African countries and the United Nations is still seeking men, helicopters and intelligence support from contributing countries before the mission is fully up and running by the year's end.
VOTE DELAY URGED
Diplomats and U.N. officials say China has already pledged peacekeepers.
Bamako has secured an agreement with separatist Tuareg rebels to allow elections to take place in areas they still occupy ahead of further talks with an elected government.
But experts and some Malian officials have expressed concern about rushing to an election that is meant to lay the foundations for rebuilding a country that was seen as a model of democracy in the region before its implosion in 2012.
Brussels-based International Crisis Group has called for a delay of no more than three months to ensure newly printed ID cards can be distributed to all voters.
"Pressing ahead within the existing timeline could lead to a chaotic and contested vote and a new president without the legitimacy essential for the country's recovery," ICG said.
Diplomats say Western nations led by France and the U.S. are pushing for the date to be maintained as the current interim administration is unable to tackle Mali's problems.
Speaking in Geneva on Monday, U.N. Secretary-General Ban Ki-moon said holding a poll on July 28 that was credible, peaceful and accepted by all Malians would be "an enormous undertaking".