A Sudanese soldier poses next to a machine gun in the oil region of Heglig
Khartoum's warplanes bombed border regions, leading South Sudan's leader on Tuesday to accuse Sudan of declaring war, as the United States condemned the "provocative" strikes.
The overnight raids, launched in defiance of global calls for restraint, wounded several people in the villages in the South's oil-rich border regions, reaching around 25 kilometres (15 miles) from the front line between the rival armies.
Salva Kiir, on a visit to Beijing where he met President Hu Jintao, said his "neighbour in Khartoum has declared war on the Republic of South Sudan."
Last week, Sudan's President Omar al-Bashir threatened to crush the "insect" government of the South, and said the time for talks was over.
Beijing -- a key Khartoum ally but also the main buyer of the South's oil -- has repeatedly called for an end to weeks of border fighting which saw the South seize and hold the Heglig oil field from Sudanese troops for 10 days.
Washington, the driving force behind South Sudan's struggle for statehood, condemned the north's incursion and urged the former civil war foes to recommit to talks.
"Those are provocative and unacceptable actions," US Secretary of State Hillary Clinton said. "South Sudan did withdraw from Heglig. It presented an opportunity for Khartoum to resume negotiations and to make real progress between North and South, and we urge both parties to undertake that as soon as possible."
Adding to the pressure for a resolution, the African Union gave Sudan and South Sudan three months to reach a peace deal or face "appropriate measures," AU security commissioner Ramtane Lamamra said Tuesday.
Despite the South's withdrawal from the key Heglig field at the weekend, both armies are reportedly reinforcing troop numbers and digging into trenches along their contested border.
Overnight Monday, bomber aircraft hit border villages in the South's Unity state following earlier air strikes on the state capital Bentiu, governor Taban Deng said.
"There are wounded people who have been evacuated to Bentiu hospital, some of them are farmers, some are soldiers," Deng told reporters in Bentiu.
Khartoum has repeatedly denied it has launched air strikes on the South, but United Nations chief Ban Ki-moon on Monday deplored the cross-border air raids and called on both nations to prevent the fighting from escalating further.
Kiir was pleading his case to Chinese authorities, but analysts said Beijing was unlikely to take sides and would keep pushing for dialogue.
Border tensions are high, although Deng said that at present "with the exception of aerial bombardment, the front line is quiet."
However, Mac Paul, the South's deputy director of military intelligence, warned he had "information from our sources the Sudanese army is mobilising for a push on Bentiu," claims that could not be confirmed.
Bentiu lies at least 60 kilometres (40 kilometres) from the front line with Sudan's army, and large numbers of Southern troops and tanks have moved into the border zone to bolster defences.
South Sudan has warned it will fight back if Sudan does not end its aerial attacks.
Southerners are furious at what they see as international inaction against Khartoum, after they complied with the demands to withdraw.
"We were asked to withdraw from Heglig: we did. They have been asked to stop the aerial bombardment and incursions into South Sudan: they have not done so," Deng said.
"We are capable of defending ourselves, including going back to Heglig. ... I think you people should take us seriously on this. The underdog can also bite."
Sudan accuses the South of supporting anti-government rebels from its conflict-hit western region of Darfur as well as those fighting in Blue Nile and South Kordofan states.
US President Barack Obama, while calling on Sudan to cease its aerial bombardments, has said South Sudan "must end its support for armed groups inside Sudan and it must cease its military actions across the border."
Kenya, which helped broker the end of Sudan's 1983-2005 civil war in which an estimated two million people died, called for a ceasefire, with President Mwai Kibaki stressing that Sudan and South Sudan "must not return to war."
The South, which split from Sudan in July 2011 following an independence referendum, denies backing opposition movements in the north, but in turn accuses Khartoum of supporting rebels in its own territory.
The recent violence is the worst since South Sudan won independence after the civil war.