A heavily armed Sudanese military convoy moved illegally into the flashpoint border district of Abyei, sparking clashes with police that left 12 people dead, its chief administrator said on Tuesday.

The fighting broke out on Sunday when a Sudanese army major insisted on entering the disputed territory after the police tried to stop his convoy of six landcruisers mounted with machine guns and more than 200 troops, Deng Arop Kuol told AFP.

"They killed a local police sergeant from the SPLA (Sudan People's Liberation Army -- the southern armed forces). I think the UN recovered another 11 bodies. It was a violent clash," said the southern-nominated official.

"They were not supposed to enter our territory according to the Abyei protocol. So there was a plan of invading," he added, referring to an annexe of the 2005 peace deal that ended two decades of civil war between north and south.

Kuol said northern elements within the joint north-south police force set up under the protocol had joined the fighting, which took place around 17 kilometres (10 miles) north of Abyei town.

He said the northern troops were now in Goli -- further north but still within the district's boundaries.

Neither the UN peacekeeping force nor the Sudanese army spokesman were immediately available for comment.

North and south have accused each other of sending large numbers of "irregular" soldiers into Abyei, in breach of a January truce which called for the withdrawal of all forces except for the special joint police units, alongside UN peacekeepers.

Deadly fighting and recriminations have flared in Abyei since January, when the district had been due to vote on its future, alongside a referendum in the south that delivered a landslide for secession.

The plebiscite was postponed indefinitely amid deadlock between north and south over who should be eligible to vote.

The United Nations and Western observers have reported that the armies on both sides have been reinforcing their positions in recent months, raising fears of the violence escalating.

Abyei's future is the most sensitive of a raft of issues which the two sides are struggling to reach agreement on ahead of international recognition of southern independence in July.

President Omar al-Bashir warned last month that if the south claims Abyei, his government may not recognise the new state.