Obama's announcement came on Wednesday after Zimbabwe's Prime Minister Morgan Tsvangirai had called for an end to Western sanctions as he tries to rebuild the country's crippled economy.

"The crisis constituted by the actions and policies of certain members of the government of Zimbabwe and other persons to undermine Zimbabwe's democratic processes or institutions has not been resolved," Obama said in a statement. "These actions and policies pose a continuing unusual and extraordinary threat to the foreign policy of the United States."

Under the administration of George W. Bush, the United States put sanctions on the government of President Robert Mugabe in 2003. The sanctions, which ban more than 250 Zimbabwean individuals and companies from doing business with the United States, would have expired on Friday if Obama had not extended them.

Mugabe has ruled Zimbabwe since it gained independence from Britain in 1980 and Tsvangirai had sought to unseat him last year but pulled out, saying his supporters were being attacked.

Last month Tsvangirai joined Mugabe in a unity government and used his first address to parliament on Wednesday to urge the international community to take note of the power-sharing agreement and loosen sanctions.

In addition to runaway inflation, Zimbabwe is plagued by an unemployment rate above 90 percent, dire food shortages and a cholera epidemic. Foreign investors have said a new democratic government and bold economic reform are needed.