A U.S. State Department official during a background briefing Thursday said the Tunisian meeting would focus on opposition efforts on the ground and plan for political transition in Syria.
Clinton leaves London for a visit to Tunisia to participate in the meetings. The State Department said the meeting was "part of our ongoing efforts with our friends, allies, and the Syrian opposition to crystallize next steps to halt the slaughter of the Syrian people and pursue a transition to democracy in Syria."
A report from the independent international commission of inquiry on Syria found the government has "manifestly failed" to protect its own people.
"The present situation risks further radicalizing the population, deepening inter-communal tensions and eroding the fabric of society," the panel found.
The panel, in a 72-page report published Thursday, said there was a "reliable body of evidence" to suggest officials at the "highest level of government" were responsible for crimes against humanity in Syria.
The panel said that of the estimated 6,000 dead in Syria, more than 500 children were killed. Most were teenagers, though some younger than 10 years old were caught up in the violence.
Damascus maintains it is dealing with an armed insurgency backed by foreign interventionists. The inquiry acknowledged some insurgent elements were to blame, but focused its criticism on the Syrian government.
A referendum for a draft constitution is planned for the weekend in Syria.