"We see a very shaky ceasefire", Foreign Minister Børge Brende told AFP in an interview in the Sudanese capital.
His comments, after talks with Sudanese officials, came as the South's government and rebels traded accusations that each had breached the ceasefire deal by attacking the other.
"Of course I'm concerned and I think what this means is we also have to establish the right monitoring tools and also verification tools so one can really assess" the extent of compliance, Brende said.
The Troika helped oversee implementation of a 2005 peace agreement which ended Sudan's 22-year civil war and ultimately led to the South's independence in 2011.
Since then, the Troika have continued working together supporting peace and development in Sudan and South Sudan, including through backing a regional-led initiative to end weeks of fighting.
Forces loyal to South Sudan's President Salva Kiir have battled a loose coalition of army defectors and ethnic militia nominally headed by sacked vice president Riek Machar.
The seven-member East African regional bloc IGAD, which includes Sudan, mediated talks in Ethiopia between South Sudan's two warring sides.
"Sudan has played a constructive role through IGAD", Brende said.
Eighteen unarmed monitors under IGAD supervision are to oversee
implementation of the ceasefire agreement.
Up to 10,000 people are believed to have been killed in the fighting. The United Nations and rights workers report horrific atrocities have been committed by both sides.
About 700,000 people have been forced from their homes in the impoverished nation, according to the UN.
Both sides insist they are committed to the deal, and the clashes reported since the agreement was signed appear to have been localised skirmishes, not large-scale assaults.
Verifying reports from across the vast and remote regions of South Sudan -- large areas of which have poor if any telephone networks -- is difficult.
Many in the country fear that even with a ceasefire pact, the conflictpitting members of Kiir's Dinka people against Machar's Nuer tribe is far from over.
Brende said it is possible for South Sudan to "get back on track again" if a ceasefire is complied with, 11 prisoners are released, and a "robust arrangement" is reached leading to a coalition government.
"But I think we have to be realistic that... we are not there at this moment," the minister said after talks with Sudan's Foreign Minister Ali Karti, Vice-President Bakri Hassan Saleh, Oil Minister Makawi Mohammed Awad, UN officials and non-governmental groups.
A key sticking point in negotiations leading to Friday's ceasefire agreement was rebel and international demands that the South Sudanese government release 11 detained officials close to Machar.
IGAD, supported by the Troika, is working towards "a situation where the ceasefire is followed by a peace agreement followed by release of prisoners and a government where you see broad representation from all the relevant ethnic groups", the minister said.
If that happens, then Norway and other members of the international community should help South Sudan rebuild, Brende said.
"I see no alternative but it's not only the international donor community that will then have to take a responsibility. Riek Machar and Salva Kiir also have to take a responsibility to creating the right environment to get a robust agreement," Brende said.
"This has been a very, very unfortunate period where you have also lost a lot of the gains that you have seen the last year."