JUBA South Sudan accused Sudan of launching a ground attack in an oil region of the newly independent state on Tuesday and said it was preparing to strike back, in rising tension that shows no sign of abating.
The scale of the fighting was not immediately clear.
Weeks of border clashes have raised concerns that South Sudan and its northern neighbor could return to all-out war after failing to resolve wrangles over oil revenues, border demarcation and other issues since the south seceded last year.
The conflict has halted nearly all oil production in the region, strangling both countries' oil dependent economies.
Philip Aguer, spokesman for South Sudan's army, the SPLA, said Sudanese forces, militias and mercenaries attacked their positions in Hofra in the oil region of Unity state, where there have been a number of strikes in the past week.
"The SPLA is pursuing them and has repulsed the attackers. They have also captured three trucks," Aguer said.
He also said the SPLA would counter what he said was a planned attack by Sudanese army forces on the disputed border towns of Jau and Paryang.
The Sudanese army spokesman was not immediately available for comment.
South Sudan has blamed Sudan's warplanes of bombing the oil region of Unity state over the past week. Sudan has denied those attacks, but said it reserves its right to use air strikes to defend its territory.
The renewed fighting and hostile rhetoric between the former civil war foes have prompted the United Nations to take up the crisis at the Security Council.
However, China and Russia, traditionally among Sudanese President Omar Hassan al-Bashir's main backers, are resisting a Western push for the council to threaten both sides with sanctions if they fail to halt the conflict.
Beijing, which has close trade relations with Khartoum and Juba, has traditionally acted as Sudan's protector on the council and for years has shielded it from U.S. and European calls for sanctions over its handling of conflicts in its western Darfur region and elsewhere in the country.
Russia is supporting China's push to water down the resolution.
MORE PEOPLE GOING HUNGRY
Pagan Amum, South Sudan's chief oil negotiator criticized China, which is also the biggest investor in Juba's oilfields, for failing to take a stern position in the dispute.
"Honestly speaking China has not succeeded," Amum said at London think-tank Chatham House
"By trying to move away from Khartoum so as to get closer to South Sudan and trying not to get too close to South Sudan so as not to cause displeasure to Khartoum ... neither Khartoum nor Juba will be happy with China."
Fighting at South Sudan's northern border, much of which is badly demarcated, is threatening to push more people into hunger, the U.N. World Food Programme said from Juba.
"The food security situation in the border states was already precarious," WFP South Sudan Country Director Chris Nikoi said in a statement. "Now the border clashes threaten to displace more people and disrupt already fragile livelihoods."
Juba and Khartoum are also at loggerheads over citizenship. Last week Sudan told 12,000 South Sudanese waiting for months at Kosti port for Nile barges to leave the camp area within a week.
Sudan had halted river traffic in March, accusing Juba of using boats to transport weapons to rebels in the north.
The International Organisation for Migration, which helps people return home, said it was concerned about the fate of the stranded southerners and urged Sudanese authorities to allow more time for transport to be organized.
"They cannot be left to fend for themselves, and the international community does not have the logistical capacity to move them all out of Kosti by the May 5 deadline," it said.
(Additional reporting by Yeganeh Torbati in London; Writing by Yara Bayoumy)