Concerns have been expressed that work linked to the construction of the proposed £160m dual carriageway between Toome and Castledawson have been carried out close to an area used by protected swans.
Friends of the Earth director James Orr said Whooper Swans are “very sensitive breeds” and claimed such work could have breached the government's own habitat assessment.
The migrating Whooper Swans are now a familiar sight in the area with hundreds arriving each in recent weeks from Iceland to feed on the surrounding grasslands.
A Department for Infrastructure spokesperson said: “All works adjacent to the Lough Beg and Lough Neagh Special Protection Area, home to the protected Whooper Swans, have been completed in accordance with our environmental commitments.”
It added that it has consulted closely with a Whooper Swan Working Group and is “content that it has fully fulfilled its obligations”.
Campaigners have also voiced concerns that the planned upgrade will cut through an area of countryside which has been dubbed 'Heaney Country' because of its association with the poet Seamus Heaney.
An environmentalist, Chris Murphy, was recently given permission by the High Court to challenge the A6 project.