WASHINGTON — The Obama administration said on Friday that it was maintaining tough oil-related sanctions against Iran even though the U.S. and five other world powers have signed an initial deal with Tehran to curb its nuclear program and prevent it from developing nuclear weapons.

President Barack Obama has forcefully defended the interim agreement and has asked Congress not to impose new sanctions on Iran while a final, more comprehensive agreement can be negotiated. While pleading with Congress not to “close the door on diplomacy,” Obama has emphasized that crippling economic sanctions imposed on Iran have not been eased.

The White House said in a statement that there appears to be a sufficient supply of non-Iranian oil to permit foreign countries to keep cutting back purchases of Iranian oil.

In response, the State Department said in a statement that it had extended — for another six months — waivers of U.S. sanctions granted to China, India, South Korea, Turkey and Taiwan for reducing their imports of oil from Iran. The exemptions mean banks and other financial institutions based in those places will not be hit with penalties under U.S. law enacted as a way of pressuring Iran to come clean about its nuclear program. Additionally, Malaysia, South Africa, Singapore and Sri Lanka also qualified again for the waivers because they no longer purchase crude oil from Iran.

Secretary of State John Kerry said the determination was evidence that the U.S. will continue to enforce its oil sanctions during the next six months as it works to reach a comprehensive agreement that would prove the Iranian nuclear program is being used for peaceful purposes.

“We will continue to aggressively enforce our sanctions over the next six months, as we work to determine whether there is a comprehensive solution that gives us confidence that the Iranian nuclear program is for exclusively peaceful purposes,” said Kerry, who was in Switzerland to negotiate the agreement with other nations.

In exchange, Iran gains some modest relief from stiff economic sanctions and a pledge from Obama that no new penalties will be levied during the six months.