South Africa said on Thursday it will withhold aid for Zimbabwe until a representative government is in place, in what appeared to be the first punitive measure by a regional country to enforce a power-sharing agreement.

The South African government said it was "extremely concerned" about Zimbabwe's political impasse, which has deepened a humanitarian crisis, and called for "mature leadership" to resolve outstanding issues.

The tough statement came as President Robert Mugabe's ruling ZANU-PF and the opposition Movement for Democratic Change (MDC) prepared to hold another round of talks in South Africa next week to seek a breakthrough while political tensions are rising.

Mugabe is trying to push through a constitutional amendment allowing him to name a cabinet, which could lead to the unravelling of a deadlocked September 15 power-sharing agreement with the opposition.

The MDC has refused to enter the government, accusing Mugabe of trying to control most of the powerful ministries. The main obstacle in the talks is the issue of who runs the home affairs ministry, which oversees the police.

"We're going to make sure that everything is done to force the parties to go back to the negotiating table," South African cabinet spokesman Themba Maseko told reporters.

The September deal had raised hopes that a new leadership would get on with the task of rescuing the ruined economy.