The road to Heglig, an oil town that South Sudan and Sudan are fighting over, is lined with discarded furniture, destroyed buses and tanks, and clusters of dead Sudanese soldiers.

South Sudan's army, known as the SPLA, moved north into Heglig earlier this month, sparking the bloodiest fighting since South Sudan broke off from Sudan last July and became the world's newest nation. A top SPLA official said the south plans to keep moving north, taking territory the south believes it owns. The crisis threatens to widen into all-out war.

An Associated Press reporter was among the first foreign journalists to reach the disputed border since fighting began two weeks ago.

As 2nd Lt. Abram Manjil Kony sped north from the South Sudan military base at the Unity State oil field, he pointed out clusters of fallen Sudanese soldiers. Birds stalked the corpses.

"Jalaba, jalaba," Kony said, meaning "Arab" and, by extension, people from Sudan, which is predominantly Arab while the south is predominantly black.

The area around Heglig produces about half of Sudan's oil, but the south lays claim to it and says its ownership is in dispute.

South Sudan army spokesman Col. Philip Aguer said that Sudan's military bombed an oil well outside Heglig on Monday and that it continued to burn Tuesday. He said Sudan forces are trying to open other fronts along the border and that southern forces are on high alert in Western Bahr el Ghazal state.

"The border is still fragile. Tension is very high. The Sudan Armed Forces continue to bomb indiscriminately most of the areas north of Unity State. This is on a daily basis, more than twice a day," Aguer said.

SPLA soldiers occupy deserted oil facilities and a Sudanese Army base in Heglig that bears signs of a hasty retreat: Military uniforms, blankets and boots litter the ground.

Farther up the road is the Heglig market, its hundreds of stalls empty. It now serves as a forward base for SPLA forces. Just beyond, through a dry forest and up a road that is believed to be mined, is the front line. Commanders of the SPLA — the Sudan People's Revolutionary Army, a holdover name from a civil war that killed millions of people — seem unconcerned about the proximity of Sudanese troops.

They're more concerned about an attack from above. Throughout the day, Antonov planes belonging to Sudan prowl the sky, occasionally dropping bombs. Sudan's Sukhoi and MiG fighter jets have also recently been used for strikes in Bentiu, the capital of Unity State, located south of Heglig inside South Sudan.

On Saturday, two Sukhoi fighters dropped bombs within 100 meters (yards) of a bridge linking Bentiu to Rubkona, home of the SPLA's 4th Division, killing four civilians and one soldier and wounding four people. The bridge was untouched, said Maj. Gen. Mac Bol, the SPLA's deputy director for military intelligence. The next day, two bombs landed near the center of Bentiu, falling just short of the residence of the governor of Unity State.

Bol said a Sudanese military aircraft was seen "hovering" over South Sudan's capital, Juba, last week, underscoring Sudan's cointrol of the skies.

In 2005, Sudan and South Sudan signed the Comprehensive Peace Agreement, ending more than two decades of civil war. As part of the truce, the two accepted the line drawn in 1956 by the departing British colonialists as the legitimate border between the two halves of Sudan. But South Sudan says former Sudanese President Jafaar Nimeiry steadily pushed those borders southward after the discovery of oil.

Brig. Gen. Makal Kuol Deng — the SPLA's field commander for the Heglig area — says his troops have been in control of Heglig for about a week. Whether the south or the north started the clashes around Heglig two weeks ago is in dispute. Both claim to have acted in self-defense.

The former Sudan military base in Heglig provides shelter for South Sudan fighters and troops from the Justice and Equality Movement, a rebel group currently battling Khartoum in the western Sudan region of Darfur. JEM recently announced an alliance with another Sudanese rebel outfit, the SPLM-North, to form the Sudan Revolutionary Front. The JEM soldiers, wearing distinctive black and brown turbans, race around in machine-gun mounted pickup trucks.

Some 70 kilometers (45 miles) north of Heglig lies Karsana, where the SPLA believes Sudanese forces are massing. A further 28 kilometers (17.5 miles) north is the town of Keliak.

Bol said South Sudan's historical border rightfully lies between Karsana and Keliak. SPLA Field Commander Makal Kuol Deng said his troops are ready to keep pushing north if given the order.

"The order comes from the 4th Division," Deng said. "If they say we go, we go. If they say we stop, we stop."

"We don't have any intention of going farther north into the areas we think are northern areas, but of course we'll go to any place that we think is within our territory," added Bol.

Sudan has promised to push back.

In New York, Sudan's U.N. Ambassador Daffa-Alla Elhag Ali Osman said last week that if South Sudan does not withdraw, Sudan will "hit deep inside the south."

At the front lines, the fighting often begins at dusk. Before the sun sets the thumping drone of Sudanese Antonovs can be heard as they circle the sky. As the circles grow tighter, SPLA soldiers scramble into foxholes, hoping to avoid the bombs that are rolled out of the cargo bay.

Crashes can be heard in the distance, far south of Heglig. Along the road, perhaps a dozen truckloads of soldiers race north, honking their horns and cheering as they head for the front.