JUBA, Sudan — The northern Sudanese Army is threatening to seize two more areas along the combustible north-south border, risking war just weeks before southern Sudan is due to split off as an independent country, Western and Sudanese officials said Sunday.

Tensions shot up last week when northern forces stormed into Abyei, the capital of a contested region that straddles the border and is claimed by both sides, the government of Khartoum in northern Sudan and the breakaway region of southern Sudan.

Now, according to a letter from the Sudanese military’s high command, the northern army, in the next few days, plans to take over Blue Nile and Southern Kordofan states, two disputed areas with a long history of conflict that are still heavily armed.

Analysts, local leaders and Western diplomats fear that if the northern army carries through on its threat to push out or forcibly disarm the thousands of fighters allied to the south in these two areas, a conflict could erupt and set off a much bigger clash between the northern and southern armies, who have been building up their arsenals for years in anticipation of war.

Malik Agar, Blue Nile’s governor, said Sunday night that northern forces had recently moved “dangerously close” to the bases of southern-allied fighters and that he didn’t think the southern forces would surrender.

“It’s like putting a cat in a corner,” Mr. Agar said. “They will fight.”

Sudan’s border is a dizzyingly complex mosaic of ethnic groups and political loyalties. It is also home to the bulk of Sudan’s crude oil and some of the most fertile land in the country, making the question of how exactly to draw a line across Sudan one of the most explosive issues the nation confronts as it prepares to split in two.

Under peace agreements signed several years ago, joint forces were supposed to patrol some of these disputed areas. The two sides had agreed that Abyei would hold a referendum to decide if it were to join the north or south, a compromise that was essentially blotted out Saturday when thousands of northern Sudanese soldiers marched into Abyei. Southern Kordofan and Blue Nile were supposed to conduct a less formal, vaguely defined “popular consultation” process that southerners say has not been completed.

Southern Sudan is just weeks away from attaining independence, a goal that has taken more than 50 years and millions of lives. The region, one of the poorest and least developed places on earth, where four out of five adults cannot read, defied expectations in January by holding an orderly, organized referendum on independence, in which nearly 99 percent voted to split off. In the past week, southern leaders have absorbed the loss of Abyei, complaining bitterly about it but deciding not to respond with military force, saying that could jeopardize all that they have fought for.

On Sunday, southern leaders indicated that they would not fight over Blue Nile or Southern Kordofan either.

“It is not our priority now to get involved in a war,” said Barnaba Marial Benjamin, the information minister for the government of southern Sudan. He also said high-level negotiations were about to begin in Khartoum, Sudan’s capital, over several of these border issues.

But what may be more dangerous this time is that there are many more southern-allied fighters stationed in Blue Nile and Southern Kordofan than there were in Abyei — tens of thousands, compared with a few hundred in Abyei who quickly retreated last weekend when faced with a clearly superior northern Sudanese force.

“The move into the Nuba in particular will be explosive,” said Eric Reeves, a professor at Smith College and one of the leading academic voices on Sudan. “The amount of weaponry and men under arms is tremendous.”

Also, these troops are in a more desperate situation. Blue Nile and Southern Kordofan are not contiguous with the south, and if the soldiers give up their weapons, they will be at the mercy of the northern Sudanese forces whom they have fought for years.

“If it were only so simple for them to move south,” Mr. Agar said. “But they are not southerners. They are from Blue Nile and they don’t have any other place to go.”

In an example of the complexities of this area, Mr. Agar is from a predominantly northern Sudanese ethnic group, Ingesena, and his state, Blue Nile, is part of northern Sudan, according to an internal boundary established before Sudan became independent on January 1, 1956. But Mr. Agar is part of the Sudan People’s Liberation Movement, the organization that fought for southern independence. He, along with countless others from his area, personally battled as guerrilla fighters for the south against the north.

Mr. Agar said that he had recently received a written order for the southern forces in his area to disarm.

According to a letter provided to the New York Times, dated May 23 and marked “Top Secret”, the northern Sudanese army will “redeploy its forces to all areas north of the 1/1/1956 borders starting from 1 June 2011.” The letter is from Ismat Abdul Rahman Zain Al-abideen, the chief of staff for the Sudanese military. Western officials have said the northern military has threatened to attack any southern-allied soldiers north of the border who do not withdraw immediately.

Northern leaders have not been shy about their intentions to unilaterally annex large swathes of contested territory, amassing an enormous force of troops, tanks and artillery pieces in the borderlands area and publicly vowing to take control of all the disputed territory north of the 1956 border, regardless if the status of some of those areas was supposed to be decided by negotiations.

Rabie A. Atti, a government spokesman in Khartoum, the northern capital, said Sunday that just like Abyei, “Blue Nile and Nuba Mountains in southern Kordofan are part of the north, and nothing else.”

Some observers believe that the north’s maneuvering, including the letter, may have roots in an unstable domestic political situation. Many northerners are upset and fearful about losing the south, especially its oil. By openly threatening military action, Sudan’s president, Omar Hassan al Bashir, may can appear to be taking a tough stance and grabbing what little disputed territory remains. Mr. Bashir himself is in a box, having been indicted by the International Criminal Court in connection to the massacres in the western region of Darfur.

“This is one way of pushing the envelope to say, “Forget you, southern Sudan, we’re going to make all the negotiations over the border final,” said one American official who works closely on Sudan.

“I seriously doubt the south will go to war over this. It’s not worth it to them,” added the official, who was not allowed to speak publicly on the matter. “But this could lead to internal turmoil. I mean, how long is the south going to take this humiliation? How long are they going to back off?”