"Fragmented demarcation, like Swiss cheese, makes it impossible to adhere to the constitution," Britto said, referring to alternative proposals advanced by the two senators and Roraima farmers.
The governor of Roraima says the Raposa-Serra do Sol reserve, about the size of Kuwait, is too big for the 19,000 Indians who live there.
Al Jazeera's Gabriel Elizondo in Brazil says rice farmers in the state have refused to give up what they say is their rightful claim to the land.
Mining, agricultural and timber officials say the tribes are an obstacle to economic development.
The Brazilian army's chief Amazon commander as well as conservative politicians have said the reservation could compromise national security by allowing armed Colombian fighters and drug traffickers to use it.
The Makuxi, Wapixana, Ingariko, Taurepang and Patamona tribes and their supporters say the reservation is being targeted, sometimes violently, by the farmers and other economic interests.