President Donald Trump on Monday said that talks for a new nuclear deal between the US and Iran would continue on Thursday.
Iran's Foreign Ministry said on its Telegram channel, however, that talks would resume on Sunday in Oman. The two nations are in discussions over a new deal that would put limits on Tehran's nuclear programme in exchange for sanctions relief.
Mr Trump cautioned that Iran has been "asking for things that you can't do" in the indirect discussions, brokered by Oman.
"They don't want to give up what they have to give up, you know what that is? They seek enrichment [of uranium]. We can't have enrichment," he said at an economic event at the White House. "They have given us their thoughts on the deal, and I said, you know, it's just not acceptable."
Washington last month submitted a proposal for a new nuclear deal, but Tehran deemed it "unacceptable". Iran is expected to present a counter-proposal soon.
"They are good negotiators but they're tough. Sometimes it can be too tough," Mr Trump said. "We're trying to make a deal so that there's no destruction and death."
The issue of Iran continuing to enrich uranium has been a major sticking point between Iranian and American negotiators. Mr Trump said last week on his Truth Social platform that "we will not allow any enrichment of uranium".
Mr Trump added that Iran was the main topic during a phone call on Monday with Israel's Prime Minister Benjamin Netanyahu.
The call came amid reports Israel was finalising plans to strike nuclear sites in the country, a threat Israel has been making for months. Last month, Mr Trump said that he warned Israel against such a move.
At the weekend, Iran claimed to have acquired a "treasure trove” of information about Israel’s nuclear weapons. Israel is widely assumed to possess such weapons, although it has never officially acknowledged it.
Mr Trump also said Iran was involved in negotiations over ending the war in Gaza.
"Gaza, right now, is in the midst of a massive negotiation between us and Hamas and Israel, and Iran actually is involved, and we'll see what's going to happen with Gaza," he said. "We want to get the hostages back."
The US has been working on mediating a new ceasefire in Gaza between Israel and Hamas, nearly three months after a fragile truce collapsed.
Since the middle of March, Israeli strikes resumed, leading to a mounting Palestinian death toll and a worsening of the enclave's already dire humanitarian situation.
