Southern Yemeni forces launched air and missile attacks on the capital, Sana, and northern troops advanced on the southern port of Aden on Friday in the worst fighting since the two Yemens united four years ago.
The latest battles blew apart what remained of the fragile thread that has held together the Arab world's poorest country since the former Marxist south and the conservative north came together.
Yemen's official Saba news agency said southern forces, numerically inferior to the army of the north but with superior air power, fired five Russian-made Scud missiles at Sana.
The missiles, launched from bases near Aden 180 miles away, caused no casualties or serious damage, it said.
A north Yemen official said the capital also came under repeated southern air attacks.
He said raiding southern warplanes had dropped bombs at random on Sana "causing damage to some civilian areas." Seven southern plans were reportedly shot down.
Amid the chaos, President Ali Abdullah Saleh issued decrees firing the country's oil minister and the governor of Aden, members of the southern Yemen Socialist Party.
North Yemeni forces said in a statement they were advancing on Aden on four fronts and would not stop until they took the former capital of South Yemen.
The fighting has trapped an estimated 1,500 Westerners in the rugged country on the southwestern tip of the Arabian Peninsula. The French navy earlier Friday evacuated about 560 foreigners.
Further evacuations were planned by France, Moscow and Washington.
The latest battles, following pitched air and artillery fights between northern and southern forces Thursday, have cut Yemen off from the rest of the world, and it is impossible to verify rival claims.
Yemen's fragile union has been under threat because of rivalry between Saleh and Vice President Ali Salem Beidh, a southerner.