WASHINGTON, May 3 (Xinhua) -- The U.S. State Department said on Friday that it has decided to extend sanction waivers on Iran's civil nuclear projects for 90 days.

"We are permitting the following nonproliferation activities to continue, for a renewable duration of 90 days," listing the Bushehr nuclear power plant, the Fordow facility, the Arak reactor and the Tehran Research Reactor, the State Department said in a statement.

"We are permitting the temporary continuation of certain ongoing nonproliferation projects that constrain Iran's nuclear activities and that help maintain the nuclear status quo in Iran until we reach a comprehensive deal that resolves Iran's proliferation threats," it added.

However, according to a separate statement issued by State Department spokesperson Morgan Ortagus, "starting May 4, assistance to expand Iran's Bushehr Nuclear Power Plant beyond the existing reactor unit could be sanctionable."

"Our policy preserves oversight of Iran's civil nuclear program," the statement read. "We reserve the right to revoke or modify our policy covering these nonproliferation activities at any time if Iran violates its nuclear obligations or commitments or we conclude that such projects no longer provide value in constraining Iranian nuclear activities."

Following the exit from the Iran nuclear deal in May last year, the Trump administration has kept piling up pressure on Iran through a series of sanctions and designations, which have been strongly opposed and criticized by Tehran.

In November, Washington snapped back sanctions on Iran's banking, energy and shipping industries, while granting 180-day waivers to eight of Iran's major oil buyers, including South Korea, Japan and Turkey.

On April 22, Secretary of State Mike Pompeo said that the U.S. sanctions will be reimposed on all countries that import oil from Iran since May 2.

Washington's new move to squeeze Iran has raised concerns that it may lead to higher oil and gasoline prices.