WASHINGTON — Secretary of State Mike Pompeo is defending the Trump administration's reimposition of sanctions against Iran from conservative critics who argue more should be done to isolate the country.

In an interview with "Fox News Sunday," Pompeo refused to reveal which countries received waivers from U.S. sanctions to continue importing Iranian oil. But he said the eight unidentified nations "need a little bit more time to get to zero." He would not rule out the Trump administration extending the waivers beyond six months

Pompeo maintained that despite the waivers, "these sanctions have already had an enormous impact." He added that Trump's policy of "maximum pressure will be fully in place as of tomorrow."

President Donald Trump removed the U.S. from the 2015 Iran nuclear deal earlier this year, triggering the sanctions being imposed Monday. Pompeo said they are designed to "alter the Iranian regime's behavior," especially its support of terrorist groups like Hezbollah.

Trump told reporters as he left the White House on Sunday for political rallies in the South that the sanctions are the strongest ever imposed by the United States.

'We'll see what happens with Iran. But they're not doing very well," he said. The president did not respond to a question about the waiver.

Pompeo said earlier on CBS' "Face the Nation" that he was "confident" Iran would not restart its nuclear program with the U.S. withdrawing from the deal.

Pompeo and Treasury Secretary Steven Mnuchin were expected to provide more details on the sanctions at a briefing Monday.