The ICG reports renewed violence against opposition supporters, and chronic delays in political reforms.
State media are biased, the criminal justice system is being used against the opposition, and the army and police are still able to thwart democracy, it says.
Zimbabwe has had two years of relative stability following the formation of a coalition government between Zanu-PF and the Movement for Democratic Change.
But political tensions have been rising on the back of President Robert Mugabe’s call for early elections.
The ICG says that without strong international pressure on Zanu-PF, the coalition deal could collapse and the country face another illegitimate vote.
Pressure has been mounting on Mugabe from his own neighbours.
The Southern African Development Community (SADC) has called on the Zimbabwe government to create conditions for peaceful political activity.
The ICG says the SADC also needs the support of the West, including Britain, the US and the EU.
There are signs that regional pressure is bearing fruit.
Unconfirmed reports say Mugabe’s party has agreed to postpone elections to next year.