By David Lewis and John Irish

DAKAR/PARIS (Reuters) - Any foreign-backed offensive to retake control of northern Mali from al Qaeda-linked Islamists will take at least six months to prepare, plans seen by Reuters show, a delay that runs counter to the expectations of many Malians.

The timing allows space for talks aimed at fragmenting the Islamist militias and stabilizing a shaky Bamako government, but it risks giving Islamist hardliners an opportunity to dig in.

"You won't have boots on the ground in northern Mali until everything is ready to go," said a Bamako-based diplomat following the situation closely who asked not to be named.

"It is quite conceivable that there will be no military action for up to a year. Any intervention will probably need to be before April or after September," the diplomat said, referring to complications arising from the mid-year rainy season.

The fall of Mali's north to Islamists, including AQIM, al Qaeda's North African wing, has carved out a safe haven for militants and international organized crime, stirring fears of attacks in West Africa and in Europe.

African leaders will later this month seek a United Nations mandate to dispatch a mainly West African force of some 4,000 to Mali tasked with rebuilding its army and then backing operations to win back the occupied desert zones in the north.

A planning document seen by Reuters, known as the Strategic Concept of Operations, provides 180 days from the time the mandate is approved for forces to deploy in Mali, and retrain and equip the nation's army which is in tatters since a March coup and the subsequent rebel seizure of the north.

French President Francois Hollande said on Thursday he was seeking a mandate for the mission by the end of this year.

Operations to retake the north will then be launched and are expected to last 120 days, the plan shows. Three more months will be needed to stabilize the situation, it says.

Helping Mali's army retake its lost territory has been complicated by the coup and subsequent rows between the military and civilians now officially in power.

DIPLOMATIC OFFENSIVE

General Carter Ham, the top U.S. military commander for Africa, said the current plan consisted of "broad outlines" that will require further detail.

"As with all plans, further detail work is required to finalize logistics planning, transportation, the details of training a Malian force (...), how do they bring together the elements of many different nations under a single command and control architecture," he said while in Paris this week for talks with French officials.

Diplomats say additional time is needed to define the role of nations like Chad, Mauritania and Algeria, which are not part of ECOWAS but are regarded as crucial to any military operation, as well as determine who will foot the mission's bill, estimated at $300-500 million.

The African battle plan, drawn up by West Africa's ECOWAS bloc and endorsed by the African Union, estimates there are 2,500-3,000 core fighters amongst the Islamists coming from Africa, Europe and Asia.

The U.S. estimates the hard-core contingent of Islamists much lower at between 800 and 1,200. The conflict has forced 400,000 Malians to flee their homes.

The European Union is planning to send 200 troops to Mali to help training. But like the U.S. and former colonial power France, which is the keenest of Western nations for military action, Brussels has ruled out a combat role.

Regional diplomats are in the meantime pushing diplomacy to try and pick apart the Islamist alliance, in which AQIM has forged ties with Malian Islamist group Ansar Dine and MUJWA, an AQIM splinter that has attracted hundreds of local and foreign recruits.

While dialogue is not being considered with AQIM or MUJWA, labeled terrorist groups by African leaders, Ansar Dine representatives have met officials from regional mediator Burkina Faso and the United Nations this week.

Some Ansar Dine members have hinted at concessions such as distancing themselves from terrorist groups and dropping demands for sharia (Islamic law) to be imposed across Mali. Others have rejected any such moves.

"There has to be time given for more moderate elements ... to get off the battlefield. That has to play out," the diplomat said.

(Writing by David Lewis; Editing by Richard Valdmanis and Robert Woodward)