U.N. Secretary-General Ban Ki-moon urged Sudan and South Sudan on Thursday to step back from the brink of war and return to the negotiating table.
Ban called on South Sudan to immediately withdraw its forces from the oil-rich Heglig area, calling their invasion "an infringement on the sovereignty of Sudan and a clearly illegal act." He called on the government of Sudan to immediately stop shelling and bombing South Sudanese territory and withdraw its forces from disputed territories including Abyei.
The secretary-general said both countries "must stop supporting proxy forces against each other."
He spoke to reporters in New York Thursday as the Arab League announced it would hold an emergency meeting over the increasing violence between the two countries.
On the ground, the south reported new skirmishes and Sudan's President Omar al-Bashir increased his threats of war toward the south.
Al-Bashir said the recent violence has "revived the spirit of jihad" in Sudan. South Sudan said it had repulsed four attacks from Sudan over a 24-hour period as fighting on the border showed no signs of slowing.
"This is not the time for war," the U.N. secretary-general told reporters at U.N. headquarters in New York. "This is a time for leadership, for engagement, for negotiation - in the name of humanity, and in the interests of the people of both countries and the region."
"The last thing the people of these two countries need is another war - a war that could claim countless lives, destroy hope and ruin the prospects of peace and stability and prosperity of all Sudanese people," Ban said.
Acting on a request by Sudan, the Arab League scheduled an emergency meeting of foreign ministers in Cairo next week to discuss the violence, Deputy Arab League Secretary-General Ahmed bin Helli said. The league earlier called on South Sudan to withdraw from the Heglig area that southern troops invaded and took over last week.
Despite the threats from Sudan, a southern government spokesman said South Sudan was only defending its territory and considers Sudan a "friendly nation."
South Sudan military spokesman Col. Philip Aguer said three of the attacks were on Wednesday and one was on Thursday. He did not give a death toll.
South Sudan broke away from Sudan last year after a self-determination vote for independence. That vote was guaranteed in a mediated end to decades of civil war between the two sides. But the sides never fully agreed where their shared border lay, nor did they reach agreement on how to share oil wealth that is pumped from the border region.
Instead, the two countries have seen a sharp increase in violence in recent weeks, especially around the oil-producing town of Heglig. Both sides claim Heglig as their own. It lies in a region where the border was never clearly defined.
Aguer said southern troops repulsed one attack by Sudanese troops near Heglig on Wednesday and two attacks in Northern Bahr el Ghazal state. One was repulsed in Western Bahr el Ghazal state early Thursday, he said.
Al-Bashir on Wednesday threatened to topple the South Sudan government after accusing the south of trying to take down his Khartoum-based government.
He continued his hardline rhetoric on Thursday in an address to a "popular defense" brigade headed to the Heglig area. The ceremony was held in al-Obeid, in northern Kordofan.
"Sudan will cut off the hand that harms it," said al-Bashir, a career army officer who fought against the southern army, the Sudan People's Liberation Army, during the 1983-2005 civil war. Al-Bashir seized power in a 1989 military coup.
The capture of Heglig by the South Sudanese "has revived the spirit of jihad and martyrdom among the Sudanese people," he told the brigade's 2,300 men, according to the official Sudan News Agency.
In Khartoum, the pro-government Sudanese Media Center said late Wednesday that fighting broke out between the two nations in the Al-Meram area in South Kordofan, with northern troops driving away what it called "remaining elements" of the SPLA. It said northern troops chased away SPLA fighters who fled across the border into South Sudan.
It said the fighting left an unspecified number of dead and wounded among the SPLA forces but gave no precise figures.
South Sudan government spokesman Barnaba Marial Benjamin said South Sudan does not consider itself at war with Sudan, but he said the south is defending territory it believes it owns based on borders outlined in 1956 by British colonialists.
"Up to now we have not crossed even an inch into Sudan," Benjamin said. He added: "The Republic of South Sudan considers the Republic of Sudan to be a neighbor and a friendly nation."
Benjamin said that southern forces would withdraw from Heglig if the African Union guarantees a cessation of hostilities, an agreement on border demarcation, and the withdrawal of Sudanese forces from the nearby border region of Abyei, with Ethiopian troops moving in as peacekeepers.
Benjamin said that al-Bashir is carrying out "genocide" against Sudanese people in the Darfur, South Kordofan and Blue Nile regions of Sudan. He said al-Bashir's words Wednesday were a warning that he would like to do the same in South Sudan.
"Can they quote one war fought by the Republic of Sudan fought with any foreign country? They have always used their military artillery to kill the innocent people of Sudan as well as South Sudan," Benjamin said.
The International Crisis Group said in a new analysis on Thursday that Sudan and South Sudan are "teetering on the brink of all-out war from which neither would benefit." It said an immediate cease-fire is needed, then solutions to the unresolved post-referendum issues.
"Increasingly angry rhetoric, support for each other's rebels, poor command and control, and brinkmanship, risk escalating limited and contained conflict into a full-scale confrontation," the group said. "Diplomatic pressure to cease hostilities and return to negotiations must be exerted on both governments by the region and the United Nations Security Council, as well as such partners as the U.S., China and key Gulf states."
U.S. special envoy to Sudan and South Sudan Princeton Lyman called the situation "a very, very serious crisis between Sudan and South Sudan" that "affects international peace and security," but said it was not yet an all-out war and expressed hope that one could be prevented.
"It's not going to be easy," Lyman said on a conference call from Khartoum, where he is having meetings after spending several days in Juba talking to officials there. "Emotions are running very, very high."
The U.S. played a large role in brokering the 2005 peace accord between the two sides. China is a big player in the two countries' oil industry.
Associated Press reporters Edith M. Lederer at the United Nations, Mohamed Saeed in Khartoum, Sudan, and Matthew Lee in Washington, contributed to this report.