President Donald Trump's special envoy for Iran further stoked tensions between Iran and the U.S. on Friday when he said the United States will sanction any country that imports Iranian crude oil.

Envoy Brian Hook, in his comments to reporters in London, said that there were no exemptions, reiterating a threat the Trump administration made two months earlier.

"We will sanction any imports of Iranian crude oil," Hook said, according to Reuters.

"There are right now no oil waivers in place," said Hook, who added the administration intends to "sanction any illicit purchases of Iranian crude oil."

Hook's comments came four days after the Trump administration announced new economic sanctions against Iran and one day after Iran Foreign Minister Javad Zarif told Trump on Twitter that "Sanctions aren't [an] alternative to war; they ARE war."

"Negotiations and threats are mutually exclusive," Zarif added.

Special envoy Hook's new statements came as representatives to the remaining parties to the 2015 nuclear accord—the U.S. ditched it last year—met in the Austrian capital.

Per The Associated Press:

At the heart of the meeting in Vienna is Iran's desire for European countries to deliver on promises of financial relief from U.S. sanctions. Iran is insisting that it wants to save the agreement and has urged the Europeans to start buying Iranian oil or give Iran a credit line to keep the accord alive.

President Trump, who's at the G20 summit in Osaka, Japan through Saturday, indicated that his administration is keeping an attack on Iran still on table.

"I think that in the end, hopefully, it's going to work out," said Trump of the conflict between the two nations. "If it does, great. And if doesn't, you'll be hearing about it."

Hook offered a similar take on Thursday in comments to AP.

Hook said that "we are not looking for any conflict in the region," but if the U.S. is attacked, "we will respond with military force."

Last week, Trump called off a strike he'd approved against Iran just minutes before it would have taken place. The Senate, on Friday, has a chance to rein in such military force.

The chamber began a vote Friday on a measure (pdf) to prevent an unauthorized war on Iran—a possibility that has some lawmakers bristling.

"A war with Iran would be a colossal mistake," said co-sponsor Sen. Tim Kaine (D-Va.) on the floor Thursday ahead of the vote.

"After 18 years of two wars in the Middle East, both of which where we still have troops deployed," said Kaine, "we should not be fomenting, encouraging, blundering toward, rushing into a third war in the Middle East."

Rep. Ro Khanna, sponsor of companion measure in the House, agreed.

In a Twitter thread Thursday, the California Democrat said that "the Senate is responding to a growing desire to avoid endless wars by voting to ensure Congress has a say before America is committed to another regime change conflict in the Middle East."

"With the House soon voting on our amendment to prevent an unauthorized war against Iran," Khanna continued, "it is clear there is bicameral, bipartisan momentum for Congress to reclaim its constitutional jurisdiction."