Clashes between separatist fighters and the military in southern Yemen have left 13 people injured, three of them soldiers, according to an official.

The fighting on Saturday comes as Yemen's government, headed by Ali Abdullah Saleh, the president, steps up a crackdown on secessionists in the south.

The soldiers were attempting to arrest suspects in the killing of a local intelligence official in the province of Dalea, the Reuters news agency quoted an unnamed official as saying.

Seven separatist leaders were reportedly arrested in the operation.

The AFP news agency also recorded the clashes, but reported that most of the injured were Yemeni policemen, citing an unnamed medical source.

Security crackdown

Witnesses and southern media said Yemeni security forces had surrounded Dalea city from all sides at dawn on Saturday and raided a number of houses, sparking clashes with armed groups.

The violence spread to many parts of the city, the news website Sahwa Net reported.

Reuters reported residents as saying an indefinite curfew had been announced for 6pm local time (15:00 GMT).

North and South Yemen formally united in 1990, but many in the south, where most of the country's oil facilities are located, complain northerners have used unification to seize resources.

Yemen has become a major security concern for Western nations, who fear al-Qaeda are using the country's instability to recruit and train fighters.

In addition to tackling secessionist sentiment in the South and fighting al-Qaeda, the government is also trying to bring an end to a Shia rebellion in the North.