Frank O'Connor, the franchise development director for Halo, said that "a lot of people at 343 are already prototyping ideas and software and art for future Halo products".
He added: "I can't say more than that, but work on the game started actually before we launched Halo 4 and continues apace as we cycle staff through the team's vacation breaks. We're excited about the next game."
O'Connor didn't explicitly state that the next game is specifically Halo 5, though Microsoft has already confirmed that it is building a second trilogy of Halo titles.
Many within the 343 Industries studio are still working on post launch content for Halo 4, the latest being the curation of a major international Halo 4 tournament.
It has also developed a plot-led series of co-op games called Spartan Ops, which has been widely praised by critics. A second season of the co-op mode, however, remains up in the air.
"Spartan Ops season 2 is yet to be decided, we're looking at how the first season does," said O'Connor.
"We got such great responses from [live-action web series] Forward Unto Dawn and Spartan Ops that we're pretty happy with it and we're learning a lot of really interesting stuff for the future," he added. "You are going to see a spectrum of Halo support though the next year and beyond."