Andrew Mwangura, from the Kenyan branch of the International Seafarers Assistance Programme, said on Thursday: "We do not know the exact date of their capture."
Local Puntland elders, working on the negotiations, said the hostages were in good health.
Mwangura said the two aid workers had been captured in a bid to obtain the release of three Finnish-registered fishing boats, seized along with nine traditional dhow boats off the Puntland coast last week.
"They are detaining them as a bargaining chip," Mwangura said.
Puntland governs itself independently from the rest of war-torn Somalia and has been relatively more peaceful in recent years.
However, Somalia has had a history of abductions and assassinations of local and foreign aid workers, particularly in the self-declared independent enclave of Somaliland which lies to the east of Puntland.
Authorities there have generally blamed Islamic fighters for attacks on foreigners.
Pirates were initially blamed for the capture of the three Finnish fishing vessels, but Mwangura said that they had been grabbed by Puntland's coastguard and were being held for carrying out illegal fishing.
In another incident, the maritime official said armed men had seized a cargo ship sailing from Dubai to Mogadishu on May 3, and it was being held at Hobyo port, north of Mogadishu.
"Negotiations between the cargo owners and the hijackers are underway," he said.
Puntland declared itself autonomous from the rest of Somalia in August 1998 under the leaderhip of Abdullahi Yusuf Ahmed, the current president of the Somali interim government.