* PM forced to resign on television after arrest by soldiers
* Military spokesman says not a coup, but PM 'blocking' progress
* Development complicates efforts to organise intervention in north
By Tiemoko Diallo and Adama Diarra
BAMAKO, Dec 11 (Reuters) - Mali's prime minister was forced to resign on Tuesday by the soldiers who staged a coup in March, underscoring the military's continuing grip and complicating international efforts to help push Islamists from the north.
Once a beacon of democracy in West Africa, Mali has been mired in crisis since the coup, when ethnic Tuareg rebels and al Qaeda-linked Islamist fighters took advantage of the chaos to seize the northern two-thirds of the arid nation.
Although the soldiers gave way to a civilian president and prime minister in April under international pressure, they have never been far from power and have shown their readiness to stamp their authority on divided and weak politicians.
Cheick Modibo Diarra resigned as prime minister hours after he was arrested trying to leave the country for former colonial power France and was brought to the ex-junta's headquarters at a barracks in Kati, just outside Bamako.
"I, Cheick Modibo Diarra, hereby resign with my entire government," a nervous-looking Diarra said in a short statement broadcast on state television early on Tuesday. Diarra is a former NASA scientist and Microsoft chief for Africa.
Fearing Mali has become a safe haven for terrorism and organised crime, West African leaders have signed off on a plan to send 3,300 soldiers to Mali to revamp Mali's army and then support operations to retake the north.
But Diarra's arrest could discourage international partners from backing the plan until civilian rule is strengthened.
"What is really clear now is that the military junta is the one that is in control," said Gilles Yabi, head of the International Crisis Group's West Africa programme.
"They have the weapons, they have the force and they are taking control of the transition," he said. "It clarifies the fact that the immediate obstacle in the crisis in Mali is now in Bamako and not the north."
France, the keenest of the foreign powers to see action against the Islamists, called on Tuesday for a new government to be set up under interim civilian President Diouncounda Traore.
"These developments underline the need for the rapid deployment of an African stabilisation force," French Foreign Ministry spokesman Philippe Lalliot told reporters in Paris, demanding that the former junta stop interfering in politics.
Diarra was forced to step down during a meeting with ex-coup leader Captain Amadou Sanogo, according to Bakary Mariko, a spokesman for the former junta. Mariko accused Diarra of urging supporters to disrupt talks on the political crisis and said he had failed to liberate the north or organise free elections.
"This is not a coup. The president is still in place but the prime minister was no longer working in the interests of the country," Mariko said.
A soldier in Kati said he witnessed a tearful Diarra being scolded by Sanogo for having brought shame to the country.
The president was due to address Malians later on Tuesday. Downtown Bamako was calm, but the main road leading to Kati was blocked for security reasons, residents said.
Sanogo has been repeatedly accused by his critics of political meddling since he stepped down and was given the task of overseeing reforms of Mali's army.
Even before Diarra's arrest and resignation, support for the military intervention plan was not universal.
Despite France's desire to see military action against Islamist groups that include al Qaeda's North African wing, AQIM, the United States and the United Nations have expressed concern, saying the plan lacks necessary detail.
The United Nations warned on Monday that Mali was "one of the potentially most explosive corners of the world".
"Discussions at the Security Council were already difficult. Now they are even more so," said a Bamako-based diplomat.
Some of Mali's politicians support the idea of a foreign-backed military operation while others, including much of the military, say they need only financial and logistical support.
The arrival of a foreign force in Mali might weaken the sway of the former junta.
Mariko said the Malian army was ready to act even without international help.
"We want the help of the international community but if it has to wait until September or until an undefined date, then the Malian army will act to free its territory," he said.
The European Union on Monday approved a plan to send 250 trainers to revamp Mali's military but stressed that the army would have to be under civilian authority.
As the son-in-law of Moussa Traore, a former Malian coup leader and president, Diarra had appeared to have good ties with the military.
However, tensions became particularly acute in recent weeks, with analysts saying Diarra, a relative newcomer to Malian politics after years abroad, seemed keen to establish a political base of his own ahead of any future elections.
"I am not sad to see him go but the involvement of the soldiers will only complicate things internationally," said Bamako resident Kassoum Togola.