The warning from Iran's lead negotiator, parliament speaker Mohammed Bager Qalibaf, laid bare the continued volatility in the region following Tuesday's ceasefire announcement by US President Donald Trump.
The two sides have laid out sharply contrasting agendas for peace talks set to start on Saturday, but it was unclear whether the two-week ceasefire would hold until then.
Qalibaf said Israel had already violated several conditions of that ceasefire by ramping up its parallel war against the Iran-aligned militia Hezbollah, while the US had violated the agreement by insisting that Iran abandon its nuclear ambitions.
"In such a situation, a bilateral ceasefire or negotiations were unreasonable," he said in a statement on Wednesday..
Israel and the US both said the two-week ceasefire did not cover Lebanon, and Israeli Prime Minister Benjamin Netanyahu said the strikes would continue.
"I think the Iranians thought that the ceasefire included Lebanon, and it just didn't," US Vice President JD Vance, who will lead the US delegation, told reporters in Budapest.
The two sides appeared to be far apart on Iran's nuclear program as well - one of the factors that Trump cited as the basis for war.
Trump said Iran had agreed to stop enriching uranium, which can be turned into nuclear weapons, and the White House said Iran has indicated it would turn over its existing stocks.
"The United States will, working with Iran, dig up and remove all of the deeply buried ... Nuclear 'Dust,'" Trump said on social media.
Qalibaf, however, said it was allowed to continue enriching uranium under the terms of the ceasefire.
Though both the US and Iran declared victory in a five-week-old war that has killed thousands of people, their core disputes remained unresolved. Each side is sticking to competing demands for a deal that could shape the Middle East for generations.
Netanyahu said Israel had its "finger on the trigger" and was prepared to return to fighting at any moment.
Lebanon's civil defence service said 254 people had been killed in Israel's strikes across Lebanon on Wednesday. The highest toll was in the capital Beirut, where Israeli strikes killed 91 people, it said.
Residents said some of the Israeli strikes had come without the usual warnings for civilians to evacuate.
Hezbollah, the Iran-backed militant group in Lebanon, said early on Thursday that it fired rockets at northern Israel in response to "ceasefire violations".
Leaders of 13 European countries, Japan and Canada also issued a joint statement welcoming the ceasefire and calling for a swift end to hostilities in order to "avert a severe global energy crisis".
Iran also struck oil facilities in nearby Gulf countries, including a pipeline in Saudi Arabia that has been used to bypass the blockaded Strait of Hormuz, according to an oil industry source. Kuwait, Bahrain and the UAE also reported missile and drone strikes.
The strait remained shut to vessels sailing without a permit and shippers said they needed more clarity before resuming transit.
Crowds took to the streets of Iran overnight to celebrate, waving Iranian flags and burning those of the US and Israel.
