The ODM is demanding constitutional changes in order to create a powerful post of prime minister for Raila Odinga, something Kibaki's Party of National Unity (PNU) has looked unlikely to concede.
As anxiety over the delays grew, the opposition accused the government of not being a "serious partner" in the talks, and of using stalling tactics to cement its hold on power.
Anyang' Nyong'o, the ODM's secretary general, said: "The ODM proposes that parliament be summoned within the next one week to enact the necessary changes in the constitution to implement these mediation proposals.
"If that does not happen ODM will resume peaceful mass action."
The opposition accuses Kibaki's team of stealing victory at the December 27 polls.
Kibaki's side says it won fairly and accuses ODM of instigating tribal violence following the final results.
Calls by Condoleezza Rice, the US secretary of state, Annan and other nations to allow a power-sharing deal or "grand coalition", have thus far failed.
On Tuesday, Kibaki said he was "willing to work together and share responsibilities in government" with ODM, but that any deal "must be in tandem with the current Kenyan constitution".
The government's insistence on sticking to the constitution - a colonial-era treaty which all sides agree is long overdue for reform - could block any special new arrangement to accommodate ODM like a premier's post for Odinga, analysts say.
Many Kenyans fear a return to bloodshed if a final deal is not struck soon, and local media say gangs in some conflict-hit areas have been re-arming with crude weapons.
On Wednesday, police said at least five people died in clashes during the last four days in western Kenya.
The United Nations has also warned of looming food shortages as the unrest affected crop planting, particularly in the Rift Valley where about half of Kenya's cereals are grown.
A deadline set by Annan for a political deal by mid-February has passed, but the former UN chief has vowed he will stay until the talks reach an "irreversible" point.
On Tuesday, Kenya Airways announced it had suspended its thrice-weekly flights to France because of a sharp decline in bookings caused by the political crisis here.
The national airline said the route, opened in October 2006, would resume only after the French government withdrew its travel advisory against visiting the east African state.
Titus Naikuni, Kenya Airways chief, said: "We will monitor the situation on a monthly basis and are hopeful that we will resume Paris flights for the summer high season once the French government's travel advisory has been removed," adding that the suspension would take effect on February 26.