Representatives of political parties and civil society groups met in the capital Ouagadougou to hammer out a handover plan, after Compaore fled following a mass uprising against his bid to revise the constitution and extend his 27-year rule.
The army's power grab in the landlocked west African country has attracted international condemnation and threats of sanctions from the African Union unless it hands over power within two weeks.
"We're counting on respect for the (army's) promise to put in place a democratic transition government which is led by a civilian," Williams said in French.
Washington and Paris, Burkina Faso's two main allies and donors, have been pressuring the military to quickly carry out elections.
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Around 60 representatives of the opposition, civil society, as well as religious and traditional groups met briefly Saturday morning and were due to meet again after consultations later in the day.
The groups have already agreed that the transition should last one year and that it should be led by a civilian before presidential and legislative elections take place by November 2015.
But there was no agreement on the person to head the transition.
The proposals were due to be presented Monday to mediators from the United Nations, the African Union and the west African regional bloc ECOWAS.
From his exile in the neighbouring Ivory Coast, Compaore meanwhile accused the opposition of plotting a coup with the army, in an interview published Saturday.
"They wanted me to leave. I left. History will tell us if they were right," said the 63-year-old who first took power in a 1987 coup.
As for Zida, Compaore said the lieutenant colonel was in a position that he would "not wish for his worst enemy.