WASHINGTON - U.S.-Iranian relations lurched downward again this week as the Trump administration unveiled sweeping new sanctions designed to tighten its stranglehold on Iran's oil exports, and Iran announced it would take further steps away from restraints on its nuclear program.

The administration took action Wednesday against a shipping network it said was directed by Iran's Islamic Revolutionary Guard Corps-Quds Force and designed to evade U.S. sanctions imposed in November on Iranian oil exports. In starkly worded warnings to international maritime and insurance interests, the United States declared that it would also sanction any individual or entity that did business with anyone connected to the network.

"Failure to take heed . . . bears grave consequences," an administration official said, speaking on the condition of anonymity under administration rules. The State Department also offered a $15 million reward for information that helps disrupt the network.

The new sanctions came 10 days after President Donald Trump and Iranian President Hassan Rouhani indicated a willingness to meet.

Trump repeated the offer Wednesday, even as White House and Treasury and State Department officials were discussing the new measures. The two leaders are to attend the annual United Nations General Assembly late this month.

"Sure. Anything is possible," Trump said of that venue. "We could solve it in 24 hours."

"We're going to see what happens. They want to talk. They want to make a deal," Trump told reporters at the White House. "We're not looking for regime change. They have tremendous potential, and I believe they are going to want to take advantage of that."

Rouhani, after first responding favorably, on Tuesday ruled out any bilateral discussions with the United States, saying Iran would talk to the administration only as part of multilateral negotiations after all sanctions have been lifted and the administration returns to the international nuclear deal from which Trump withdrew last year.

"We've said it before and we will say it again: We have no intention to hold bilateral talks with the United States. We never did and never will," Rouhani said. He said Iran will abandon restrictions on nuclear research and development, including on the advancement of centrifuges used to enrich uranium, as the next step toward reducing its commitments under the 2015 nuclear deal.

Both sides appeared to be sending mixed messages, said Dennis Ross, a Middle East expert who advised Presidents Barack Obama and George H.W. Bush.

"Below Trump, there is one consistent set of messaging - we're going to keep squeezing you until you realize that you have to come to the table. There is no give from us," he said. But "at Trump's level, the messaging is rather different."

When strict U.S. oil sanctions were imposed last fall, five countries that depended heavily on Iranian oil - China, India, Japan, South Korea and Turkey - were given temporary waivers. Those waivers were ended in May, a decision the White House said was "intended to bring Iran's oil exports down to zero, denying the regime its principal source of revenue."

With exports slashed more than 80 percent, the Iranian economy is believed to be close to collapse. As a result, the administration contends that has cut Iran's financial and weapons support to proxy groups that the United States has designated as terrorist, including Hezbollah in Lebanon and Hamas in Gaza, both of them avowed enemies of U.S. ally Israel.

The measures announced Wednesday - designating 16 entities in Iran and other countries, and 10 individuals and 11 tanker vessels - directly target the Quds Force, which the administration says is responsible for setting up an elaborate system of cutouts and shell companies to evade oil sanctions and is using the money to support terrorism.