The United States is tightening its economic sanctions on Iran by ending a set of waivers Thursday that had allowed some of the country’s largest oil buyers to continue their purchases.

With the expiration of waivers for eight buyers, Secretary of State Mike Pompeo says the United States would be exerting “maximum pressure” on the Iranian government.

The U.S. State Department called the move a fulfillment of the Trump administration’s promise “to get Iran’s oil exports to zero and deny the regime the revenue it needs to fund terrorism and violent wars abroad.”

The move is the latest in a series of steps the United States has taken since President Trump took office with a pledge to withdraw from the international agreement that limited Iran’s nuclear program in exchange for relief from economic sanctions.

Trump announced the withdrawal in May 2018, and new sanctions went into place in November, with oil purchase waivers in place for China, India, Japan, South Korea, Taiwan, Turkey, Italy and Greece.

Since then, Italy, Greece and Taiwan have halted their Iranian oil imports.

Iranian Oil Minister Bijan Zanganeh said Wednesday that the United States will not be able to bring Iranian oil exports to zero, and that Iran’s oil-producing neighbors have exaggerated their ability to increase their production to a level that would replace the Iranian oil being blocked from the market.

Since announcing last month its intention to end the oil sanctions waivers, the Trump administration has expressed confidence that countries such as Saudi Arabia and the United Arab Emirates would offset any loss in Iranian oil.

Turkey and China have attacked the U.S. action, but it is not clear whether they will continue to buy Iranian oil.