South Sudan said on Monday Sudanese war planes bombed an oil region in the newly independent state, a day after Khartoum declared a state of emergency in some border areas as tensions showed no signs of abating.
Weeks of border fighting have raised fears Sudan and South Sudan could return to all-out war, after failing to resolve a string of disputes over oil revenues and border demarcation.
Philip Aguer, spokesman for South Sudan's army, the SPLA, said Sudanese forces had bombed Panakuac in Unity State.
"There was bombing in Panakuac yesterday. Not less than four bombs were dropped," Aguer said, adding there had been no reports of casualties.
There was no immediate comment from the Sudanese army.
South Sudan has accused Sudan of using its warplanes to bomb its territories. Khartoum has denied it, though it has said it reserves the right to use air strikes in self-defence.
Unity State has come under repeated bombardment over the past week, and an air strike in its capital Bentiu last Monday killed two people.
Sudanese President Omar Hassan al-Bashir on Sunday declared a state of emergency in some areas of South Kordofan, White Nile and Sinnar provinces bordering South Sudan.
Meanwhile, Foreign Minister Ali Ahmed Karti said Sudan reserved the right to deploy its forces along the border with South Sudan for legitimate protection.
"This is within the borders of Sudan and not outside of Sudan and this is our right, we can deploy our forces anywhere," he said in Moscow after meeting his Russian counterpart.
"We're not at all preparing ourselves for war."
Further raising tensions was Sudan's arrest of a Briton, Norwegian and South African who it said had illegally entered the disputed Heglig area to spy for the SPLA.
South Sudanese officials have denied these allegations and said the men had been working with United Nations and aid groups clearing mines and had got lost in the remote territory.
The U.N. mission in South Sudan (UMISS) which said one of its officials had been taken to Khartoum with the three other men, was trying to free the group.
"UNMISS has been in contact with the Sudanese authorities to try and secure their release," said Josephine Guerrero, a spokeswoman for the mission.
The Sudanese foreign ministry said it had held discussions with the ambassadors of the countries of those arrested. In a statement, it said it told the ambassadors that the three were being investigated because they entered Sudan illegally.
"They were in areas of military activity, they possessed military equipment," the statement said, adding that the detainees were being treated in accordance with the standards of international law and the investigation would be speedy.