Haass, who served as U.S. special envoy to Northern Ireland from 2001 to 2003, was invited to return by the province's government in July after an uptick in sectarian violence.
He returned early from Christmas vacation in an effort to get a speedy agreement, the Belfast Telegraph reported.
"At some point we have got to fish or cut bait," Haass said. "That time has come."
A decision by the Belfast City Council in 2012 to limit the number of days the British flag may be flown at City Hall set off loyalist rioting. Another decision this summer to reroute a loyalist parade set off more violence.
Dissident republicans who do not accept the peace agreement have been more active, with a number of recent attempted bombings.
Meghan O'Sullivan, a Harvard professor who worked in Iraq after the 2003 U.S. invasion, is working with Haass in Belfast.
Haass said he has scheduled 24 meetings during the weekend with a final session to start at 6 a.m. Monday.