BAMAKO (Reuters) - Islamist rebels in control of northern Mali pushed south, close to government positions on Monday, army sources said, raising fears of fresh clashes after months of tense standoff.

Militants seized the northern two-thirds of the arid African nation in April, setting up what Western and regional powers fear could provide a haven for radicals to plot international attacks.

Heavily armed Islamist groups in convoys of pick-up trucks had been reported in the Mopti region, where government troops have been stationed since the revolt, a Malian military official told Reuters.

"The rebels have been advancing and have been sighted in several places ... We are waiting for them. If they attack us, we will fight back," said the official, who asked not to be identified.

Ansar Dine, one of the main rebel forces which announced it was ending a ceasefire with the government late last month, refused to comment on whether it was moving its fighters or planning an attack.

"For strategic reasons, we do not say where our troops are. The Malian government is responsible for whatever it is saying about troop movement," said group spokesman Sanda Ould Boumama.

Malian Defense Minister Colonel Yamoussa Camara told Radio France International that "jihadist groups" had deployed at several points along the demarcation line separating the vast rebel-held desert north from the government-controlled south.

Regional leaders and the African Union have been pushing a two-pronged effort of trying to negotiate a peace deal with some moderate Islamic groups and the ethnic Tuareg separatists that started the rebellion, while at the same time pushing for a military reconquest of northern Mali.

The U.N. Security Council in December approved plans to deploy an African-led military force to northern Mali to oust al Qaeda-linked groups, in an operation not expected to begin before September.

Ansar Dine has said it will not compromise in its demands for sharia, Islamic law, and autonomy for northern Mali but President Dioncounda Traore has insisted that Mali must retain its territorial unity and the secular basis of the state.

Once a beacon of democracy in West Africa, Mali has been in crisis since an army coup in March 2012 which created a power vacuum that allowed Tuareg rebels to seize the north of the country, where they want an independent state, an uprising that was hijacked by Islamists who initially fought alongside them.

(Reporting by Tiemoko Diallo; Additional reporting and writing by Bate Felix; Editing by Daniel Flynn and Robin Pomeroy)