Khamenei issued the message as funeral ceremonies were held for his father.
President Donald Trump announced "major combat operations" against Iran on Feb. 28, with massive joint U.S.-Israeli strikes targeting military, government and infrastructure sites.
Delegations from the U.S. and Iran entered negotiations last month aimed at a war-ending deal based on a memorandum of understanding signed by both countries.
The U.S. and Iran nonetheless exchanged limited strikes in late June despite the signing of the memorandum and amid the continuation of peace talks.
President Donald Trump said "1000 Missiles are Locked and Loaded and aimed at Iran," should the government attempt to assassinate him.
In a post on his social media platform, Trump said the U.S. military is "ready, willing, and able, for a one year period of time, subject to extension, to completely decimate and destroy all areas of Iran."
A U.S. official said that the question of nuclear dust "still very much is on the table" -- using a phrase to describe Iran's highly enriched uranium that remains covered in rubble after U.S. airstrikes.
"Either they're going to give us the nuclear dust or we have very low-cost military options to ensure that it remains buried underground forever," one U.S. official said.
The U.S. official later added that the nuclear materials are key to the deal.
"We have a lot of options if they resist giving the dust. There are continued military, diplomatic, economic leverage points. That's my point there, is that the United States fundamentally has the cards. We want the dust," the U.S. official said.
"But I want to be clear here that if we don't get the dust, we do not have a deal with Iran," the U.S. official said.
-ABC News' Michelle Stoddart
Senior U.S. officials say that the U.S. is demanding that Iran issue a public statement that "acknowledges all channels of the Strait of Hormuz are open and they're not shooting at ships anymore."
A U.S. official said that one of the key results they're expecting out of talks set for Saturday is that Iran will agree that the Strait of Hormuz will be open in the same way it was before the conflict.
"And we are expecting that as a result of the meeting in Oman tomorrow, that that will be their position. And if it's not their position, then it's not going to be a great day for them," the official later added.
-ABC News' Michelle Stoddard
Senior U.S. officials say negotiations between the U.S. and Iran are set to continue after Iranian officials privately acknowledged firing on ships in the Strait of Hormuz was a "mistake."
"They have come back to us and asked for further talks to try to settle some of the issues," the official said. "We are hoping to get to a place where they publicly say that they will stop shooting at ships and sort of explicitly or at least implicitly acknowledging that they screwed up. We're working on that now."
The senior U.S. official later added, "So they came back to the table and said, 'We screwed up. We made a mistake. Let's keep talking.'"
The U.S. official added that Trump "has directed" teams to talk but added if Iran "keep on shooting at ships or if they engage in any other hostile act, then we're going to hit them back."
The other U.S. official also offered insight into those strikes on ships in the Strait of Hormuz, saying it came from "an errant part" of the Iranian system that "was trying to undermine the deal."
Another official noted that the strikes illustrated the fractured Iranian political system.
"We have a lot of options if the hardliners get the upper hand. But we continue to have some confidence that the rational people in their system will be able to rein in those hardliners. You never know. You can't predict the future," the U.S. official said.
Despite those increased hostilities, a different senior U.S. official said that they think, "Iran is showing a lot of signs of wanting to make this deal."
-ABC News' Michelle Stoddart
Editor's note: This post has been updated
