Sudan's powerful intelligence service on Thursday said it had disrupted a "plot" by members of the military and the political opposition to disturb the country's security.
At about the same time a witness saw tanks moving in the streets of the capital.
"The security and intelligence service early today stopped a plot to disturb security," said the Sudanese Media Centre, which is close to the security apparatus.
The Media Centre quoted a source as saying authorities had been investigating both civilian and military personnel linked to the security disturbance.
"This plot is led by some opposition party leaders," the Media Centre said.
A witness told AFP that he saw troops moving early Thursday, about the same time as the plot was reportedly broken up.
"About 2:00 am (2300 GMT) while passing Obeid Khatim Street I saw some tanks and vehicles with military equipment and soldiers coming from a southerly direction and heading downtown," said the witness, who asked for anonymity.
Obeid Khatim Street is a wide thoroughfare running alongside Khartoum's military and civilian airports, leading into the downtown area where government buildings are located.
However, there were no signs of extra troops downtown later Thursday.
Sudan's opposition has been seeking a peaceful end to the 23-year regime of President Omar al-Bashir, who took power in an Islamist-backed coup.
Islamist reformers charge that corruption and other problems have left the vast African nation's government Islamic only in name and question how much longer Bashir should remain in power.
"A lot of people are saying 23 years is too long a time, and what's the difference between him and Mubarak and Assad?" a Sudan analyst who asked for anonymity said earlier.
He was referring to ousted Egyptian president Hosni Mubarak and Syria's President Bashar al-Assad.
In June and July scattered youth-driven protests, initially sparked by high inflation, called for an end to the regime. The protests petered out in the face of widespread arrests.
The report of the "plot" comes as concern grows over delays in implementing security and oil deals which the leaders of Sudan and South Sudan hailed in September as ending conflict, after they fought along their undemarcated border in March and April.
Sudan's army on Wednesday confirmed it attacked an area near the South Sudanese border where Darfur rebels had set up a compound, but South Sudan said bombs landed on its territory, killing five people.
"We attacked Al-Regaibat which is 40 kilometres (25 miles) north of the international border with South Sudan and 10 kilometres north of Samaha," army spokesman Sawarmi Khaled Saad said in a written statement.
Saad alleged that the rebels must have had "great support" from neighbouring South Sudan, an accusation which goes to the heart of tensions between the two nations.
"There are five casualties, civilians: Two women, one man and two children," South Sudan's army spokesman Philip Aguer told AFP.
He said the casualties came on Tuesday when bombs fell in Kiir Adem, an area near Samaha and claimed by South Sudan, which became independent from the north in July 2011.
"It's not a disputed area. It's near to the border", Aguer said, accusing Sudan of more air raids even further south on Wednesday.
But Sudan's army spokesman insisted that "we conducted our battle deep inside Sudan."
Earlier Wednesday the Justice and Equality Movement (JEM) rebels from Sudan's far-west Darfur region accused the government of bombing around the disputed border region, after the army had threatened to use force.
JEM is part of an alliance of ethnic-minority Sudanese insurgents seeking to overthrow the Arab-dominated Khartoum regime.© ANP/AFP