The International Crisis Group said in a report that Zimbabwe is turning a corner after long-time President Robert Mugabe and his erstwhile rival, the new Prime Minister Morgan Tsvangirai, formed a unity government in February.
"If the international community stands back with a wait-and-see attitude, the unity government is likely to fail. Mugabe and the military establishment will entrench themselves again," said Donald Steinberg, the group’s deputy president.
South African negotiators should also try to persuade hardliners in the military to retire before new elections are held, in exchange for limited immunity from prosecution for political crimes, the report said.
Such efforts would counter the risk of an attack against Tsvangirai or his Movement for Democratic Change party, it said.
Western countries have so far refused to extend major new aid and investments, or lift sanctions against Mugabe and his inner circle, until the unity government shows more tangible signs of progress.
The report said it was still premature for the United States, the European Union and others to remove their visa bans and asset freezes against Mugabe and his allies, or to give the government direct budget support.
But it called for more dynamic humanitarian aid to the country, where more than half the population depends on foreign food aid for survival.
Donors should help revive the education, health and sanitation systems, which are all in tatters after a decade of economic free-fall.
"Zimbabwe should be treated as a post-conflict society," the report said, saying donors should also help rebuild infrastructure and the civil service.
The unity government says it needs $8.5bn over three years to revive the economy, but so far has not won any major commitments.