Yemeni rebels detained seven people working for the Norwegian Refugee Council in the Red Sea district of Hudaida.
The aid workers were taken while distributing humanitarian relief in rebel-held Hali district on February 14 and were still being held.
They were seized over accusations that they accepted and distributed aid from the Saudi-led coalition that has battled the Houthi rebels since March 2015.
However, the Norwegian Refugee Council said no aid from the coalition was being delivered and a misunderstanding had taken place.
"We stress that, as humanitarians, we do not take any sides in any areas of the conflicts where we work," the group said in a statement to Al Jazeera.
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Boxes containing humanitarian aid had been recycled after use by the Arab coalition two years ago, but still had its military markings on the inside, the Norwegian Refugee Council explained. Recycling materials is common in the impoverished country.
"We reiterate that NRC does not have any Saudi-funded projects in Yemen. Due to the security sensitivities regarding our staff, we cannot comment any further on this matter at this time."
Yemen's conflict pits a Saudi-led Arab coalition supportive of President Abd-Rabbu Mansour Hadi against Iran-backed Houthis who currently control the Red Sea port of Hudaida, along with the capital Sanaa and large parts of northern Yemen.
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Monday's news comes amid a push by forces loyal to Hadi, who is backed by the Arab coalition, to close in on Hudaida on Yemen's western coast.
Forces loyal to the government took full control of Mokha, south of Hudaida, earlier in February as part of a major offensive to oust the Houthis and their allies from Yemen's southwestern coast.
Yemen's conflict escalated in March 2015 when the Saudi-led coalition began air strikes to help forces loyal to Hadi to take large parts of the country back from the rebels.
UN humanitarian coordinator for Yemen, James McGoldrick, last month said more than 10,000 civilians had been killed since 2015.
Source: News agencies