The two Sudans have swapped accusations of continuing to fight as a UN deadline has passed for them to cease hostilities or face sanctions.
South Sudan says Khartoum is continuing to bomb its territory - charges it denies.
Sudan says that until the South withdraws from territory it has occupied it has not "stopped hostilities".
Both sides have promised to comply with a UN Security Council resolution.
The two-day UN ultimatum was passed on Wednesday and expired at 15:00 GMT on Friday, amid fears of an all-out war between the neighbours.
The Security Council backed an African Union plan called for a written commitment by both governments to stop fighting, and threatened sanctions, such as asset freezes and travel bans, if its terms were not met.
South Sudan has already said it accepts the terms of the roadmap.
Under the roadmap, the two countries have until next Tuesday to restart negotiations and three months to reach an agreement.
Meanwhile, the International Organisation for Migration (IOM) has confirmed that the Sudanese government has extended the deadline for the return of thousands of South Sudanese refugees to their home country until 20 May - they had been given until 5 May.High tension
In a statement, Sudan's foreign ministry spokesman said Sudan would "fully commit to what has been issued in the resolution about stopping hostilities with South Sudan according to the time limits issued".
But Sudanese army spokesman Sawarmi Khaled Saad said that the South must withdraw from areas it had occupied otherwise "this means they haven't stopped hostilities", reports the AFP news agency.
South Sudan's army (SPLA) spokesman Philip Aguer told the BBC the border state of Unity had again been shelled on Friday morning, after first being attacked on Thursday afternoon.
A woman and child are now in a critical condition in Bentiu hospital after being wounded when an Antonov plane dropped 12 bombs in Lalop, he told the BBC's Focus on Africa programme.
Army base Teshwein was also shelled - and again on Friday morning with shells hitting "deep into SPLA positions", he said.
Sudan's information ministry denied the allegations, Reuters news agency reports.
Col Aguer said the forces would only react and break the ceasefire if they faced a ground offensive.
"We'll protect ourselves if we're attacked," he said. "Only Jesus Christ didn't react, but human beings like us will react."
"We'll see in the next 48 hours whether Khartoum will be credible this time."
The BBC's James Copnall in Sudan's capital, Khartoum, says Sudan pointed out in its statement the numerous ways in which it considers it has been attacked by South Sudan in the last few days.
But our reporter says both sides appear to have been brought back to the negotiating table by committing themselves to the roadmap, despite the high tension.Travel by air
Some 15,000 South Sudanese people are stranded in the Sudanese border town of Kosti in White Nile state.
Last month, Khartoum told South Sudanese living in the north - estimated to number about 500,000 - to either regularise their residency papers or leave the country, so thousands of them began their journey home.
But they have become stuck at the river port of Kosti as the authorities refused to allow barges to go and take them across the river because of concerns that the boats would carry military equipment or militia northwards.
A spokesman for the IOM in Geneva, Jean Philippe Chauzy, told the BBC that the deadline had been extended to give the IOM time to organise a land and air bridge for those stranded.
Mr Chauzy said he expected the first transports by bus to Khartoum and then by air on to Juba, to begin "quite soon".
The rights of each other's citizens is one of a number of issues still to be resolved by both countries since South Sudan became independent in July last year.
Sharing the oil revenue is the biggest one, which has led the two countries to the brink of all-out conflict.
The crisis came to a head in April, after months of border skirmishes, when South Sudan seized an oil field at Heglig, which is internationally accepted to be in Sudan, saying the area was being used as a base for Sudanese attacks on its territory.
South Sudan took most of the oil reserves when it seceded, but relies on pipelines to seaports in Sudan for distribution.
In January, it decided to shut down oil production, which provides 98% of the government's revenue, after Khartoum impounded South Sudanese oil shipments amid a dispute over transit fees.
Last year Southern Sudanese voted overwhelming in favour of independence from Sudan in a referendum promised as part of a peace deal in 2005 which put an end to the 22-year civil war in which some 1.5m people died.
Sudan: A country divided
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.