"We see a very shaky ceasefire", Foreign Minister Borge Brende said in an interview yesterday 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.
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.
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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.
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.
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.