BAGHDAD — Sunni militants consolidated and extended their control over northern Iraq on Wednesday, seizing Tikrit, the hometown of Saddam Hussein, threatening the strategic oil refining town of Baiji and pushing south toward Baghdad, their ultimate target, Iraqi sources said.

As the dimensions of the assault began to become clear, it was evident that a number of militant groups had joined forces, including Baathist military commanders from the Hussein era, whose goal is to rout the government of Prime Minister Nuri Kamal al-Maliki. One of the Baathists, Izzat Ibrahim al-Douri, was a top military commander and a vice president in the Hussein government and one of the few prominent Baathists to evade capture by the Americans throughout the occupation.

“These groups were unified by the same goal, which is getting rid of this sectarian government, ending this corrupt army and negotiating to form the Sunni Region,” said Abu Karam, a senior Baathist leader and a former high-ranking army officer, who said planning for the offensive had begun two years ago. “The decisive battle will be in northern Baghdad. These groups will not stop in Tikrit and will keep moving toward Baghdad.”