Representatives of both men continue to meet in South Africa in an attempt to form a national unity government, though Mr. Mugabe’s opponents remain deeply skeptical that he intends to share power after a bloody election campaign that included the beatings and killings of opposition supporters.

In his statement, Mr. Bush sought to offer both a carrot and a stick. He said that the United States would offer “a substantial assistance package” to the impoverished and politically tumultuous country if a representative government took office, presumably without Mr. Mugabe as president.

Separately, he ordered the State Department to free up $2.5 million in emergency assistance for refugees and people seeking political asylum from Mr. Mugabe’s government. He said that the United States would continue to contribute humanitarian aid.

Arvind Ganesan, director of business and human rights for Human Rights Watch, said that the impact of the sanctions would depend on how aggressively the United States enforces them and whether other international corporations also abide by them, even if not required by law to do so. He called that “the chilling effect.”

“It signals the administration is getting frustrated by the slow reaction of the international community,” he said.

Before Friday’s action, the United States already imposed sanctions against Mr. Mugabe, the country’s president for 28 years, and 129 other individuals and 35 businesses. Those sanctions freeze all assets in banks within United States jurisdiction and prohibit Americans from trading with the individuals or businesses. The American sanctions against Zimbabwe first went into effect in 2003 and were expanded in 2005.

Friday’s order added 17 more businesses, which the Treasury Department said in an unusually pointed statement were used by Mr. Mugabe and his “regime cronies” to “illegally siphon revenue and foreign exchange from the Zimbabwean people.”