In a letter released Tuesday, Duncan urged Spence to end her hunger strike and said his office has made attempts to set up a meeting with her but received no response.
"It is unfortunate that you are unwilling to speak with me about the issues you have raised publicly," his letter says.
"I remain concerned about your health and hope that you will accept my offer to speak about how we might move forward with improving the treaty relationship."
Protesters block CN Rail line
Meanwhile, scores of First Nations protesters in Sarnia, Ont., say their demonstration will continue until Prime Minister Stephen Harper meets with Spence.
Ron Plain of the Aamjiwnaang First Nation says donations such as blankets and food are coming in from within the province and as far away as California as word of the demonstration spreads through social media.
Plain says organizers have yet to decide whether to challenge a court injunction granting police the power to end the protest. Sarnia police have said they won't move to stop the blockade unless there is a safety risk.
Plain says there is an "air of excitement" at the Sarnia blockade, with Aamjiwnaang youth — who started the protest Friday as part of the national Idle No More movement — building snowmen along the tracks.
"The resolve of the community seems to be deepening," Plain said, adding the demonstration is "turning into a community event as opposed to a blockade."
The protests are part of the national Idle No More movement against the government's legislation.
Plain said a representative for the rail company spoke with organizers Monday, but the talks were not fruitful.
He said the First Nation is unwavering in its stance that the tracks were not laid legitimately.
"Our view is a very solid view in that the tracks are not permitted on that road. There was never any kind of permit issued for those tracks to cross there."
Spence started her hunger strike on Dec. 11, and has been living in a teepee on an island in the Ottawa River that many aboriginals consider to be sacred land.
She is seeking a meeting with Harper, the governor general and First Nations leaders to discuss the treaty relationship.
Senator Brazeau also rebuffed
On Monday, Conservative Senator Patrick Brazeau, who is Algonquin, was rebuffed in an attempt to meet with her.
Last week, Brazeau told the Aboriginal Peoples Television Network he didn't think Spence was setting a good example for aboriginal youth.
Brazeau posted an account on Twitter of his efforts to meet Spence on Monday, saying he was first told she needed time to prepare to see him but then that she wouldn't see him at all.
"Not sure if Chief Spence's advisors made the recommendation not to meet but regardless, I respect her decision," Brazeau wrote.