Iran's military scoffed at the diplomatic effort and launched more attacks on Israel and the Gulf region, including an assault that sparked a fire at Kuwait International Airport.
With growing pressure on the United States to end the war as it nears its first month, Washington submitted the plan to Iran by intermediaries from Pakistan who have offered to host renewed negotiations.
Iran's attacks on regional energy infrastructure and its chokehold on the Strait of Hormuz, through which a fifth of the world's oil is usually shipped, has sent oil prices skyrocketing and shaken world markets over fears of a global energy crisis.
Despite reports of negotiations, the Pentagon is expected to send thousands of soldiers from the US Army's elite 82nd Airborne Division to the Middle East, adding to a massive US military build-up.
The forces will add to the 50,000 US troops already in the region.
The moves are being framed as US President Donald Trump allowing himself "max flexibility" on what he would do next.
Iran's unified command for the military and Revolutionary Guard suggested there were no talks.
"Have your internal conflicts reached the point where you are negotiating with yourselves?" spokesman Ebrahim Zolfaghari said.
"Our first and last word has been the same from day one, and it will stay that way: someone like us will never come to terms with someone like you," the lieutenant colonel said in the video statement aired on state television.
"Not now, not ever."
Israeli officials, who have called for Trump to continue the war against Iran, were surprised by the submission of a ceasefire plan, an official said.
The White House did not respond to requests for comment.
The Israeli military announced it had begun new wide-scale attacks early on Wednesday on Iran targeting government infrastructure, and witnesses reported air strikes in the northwestern city of Qazvin.
Missile alert sirens began early in the morning in Israel as Iran launched its own attacks, which have been a daily occurrence since Israel and the US attacked Iran on February 28 to start the war.
Iran also kept up the pressure on its Gulf Arab neighbours, with Kuwait and Saudi Arabia saying they had repelled fresh drone attacks.
Any talks between the US and Iran would face monumental challenges.
Many of Washington's shifting objectives, particularly over Iran's ballistic missile and nuclear programs, remain difficult to achieve.
It's not clear who in Iran's government has the authority to negotiate - or would be willing to, as Israel has vowed to continue killing the country's leaders.
Iran remains highly suspicious of the United States, which twice under the Trump administration has attacked during high-level diplomatic talks, including with the strikes that started the current war.
Zolfaghari said the US was in no position to negotiate: "The one claiming to be a global superpower would have already gotten out of this mess if it could."
Speaking at the White House, the president said the US was "in negotiations right now" and the participants included special envoy Steve Witkoff, his son-in-law Jared Kushner, Secretary of State Marco Rubio and Vice-President JD Vance.
"And the other side, I can tell you, they'd like to make a deal," Trump said.
Iranian Foreign Minister Abbas Araghchi's office said he had been discussing the war with several counterparts, but parliament speaker Mohammad Bagher Qalibaf denied Trump's claim of direct talks.
with Reuters
