The tank battle at Amran, 50km (30 miles) from Sanaa, capital of former North Yemen and seat of government of unified Yemen, is the most serious fighting between northern and southern troops since the two countries merged in 1990.
Tensions between north and south are more than mere personality differences between President Ali Abdullah Salih, from the north, and the southern leader, Ali Salim al-Beidh, the Vice-President. The army was the most powerful institution not to have been integrated. Last year the chief of staff resigned over the non- integration of the two armies.
By arrangement, northern forces have been stationed in southern areas, and southern forces garrisoned in the north. This has exacerbated tensions.
The south started going its own way in the autumn, with southern ministers refusing to attend cabinet meetings in Sanaa. Mr Beidh has remained mainly in Aden, in the south. A reconciliation accord, signed in Amman in February, has not been implemented.