A day after Israeli and US leaders sent conciliatory messages to Tehran in honor of the Nowruz new year’s holiday, Iranian Supreme Leader Ali Khamenei marked the event by questioning the Holocaust and vowing to overcome US-led sanctions.

Speaking at Mashhad, in northeastern Iran, Khamenei also intimated that Israeli Palestinian peace talks were a US-Israeli plot to cleanse “Palestine” of Muslims and Christians.

“The Holocaust is an event whose reality is uncertain and if it has happened, it’s uncertain how it has happened,” Khamenei said during his address, according to a Twitter account under his name thought to be run by his office.

“Expressing opinion about the Holocaust, or casting doubt on it, is one of the greatest sins in the West. They prevent this, arrest the doubters, try them while claiming to be a free country,” said Khamenei, who has repeatedly called the Holocaust a “myth.”

“They passionately defend their red lines … How do they expect us to overlook our red lines that are based on our revolutionary and religious beliefs.”

Officials in Iran have been reluctant to admit the slaughter of 6 million Jews in Europe at the hands of the Nazis during World War II, though moderates who took power under Khamenei in August have given signs of a shift in policy.

In September, newly elected President Hassan Rouhani appeared to repudiate official Iranian rejection of the Holocaust’s existence in an interview with CNN.

However his office later claimed that his quote had been mistranslated and he had only confirmed a crime had taken place while dodging specifics.

In February, Iranian Foreign Minister Mohammad Javad Zarif was called before Iran’s parliament for questioning, after acknowledging on German television that the Holocaust occurred.

Khamenei on Friday also called for Iranians to not wait for sanctions to be lifted but work to build a stronger economy to “reduce vulnerability.”

A nation is not powerful only because of arms, he said, according to the Fars news agency: “If three elements of economy, culture and science are in a country, it will become strong… That’s my word today: Iranian nation should make itself strong. If a nation is not powerful, it will be subject to force; the world bullying powers will receive blackmail from it; they will trample upon it and invade and insult on it,” he added.

“If we make our resolve firm and join hands, we will be able to flourish our economy. We should not pin hopes in enemy when to lift sanctions. We should look what should ourselves can do,” he also said, according to the state-run IRNA news outlet.

Iran has been hit hard by international sanctions over its nuclear program that have targeted the vital oil and financial sectors. Tehran denies it’s trying to make nuclear weapons.

Khamenei in February ordered the government to create an “economy of resistance” to counter the sanctions.

US President Barack Obama sent a video message to Iran on Thursday to mark Nowruz, saying a deal to curb nuclear activities for eased sanctions would open up “new possibilities and prosperity” for the Iranian people.

The supreme leader also lashed out at the US and Israel, which he said were trying to turn “Palestine” into a place where Christians and Muslims cannot live, an apparent reference to ongoing US-led peace talks.

He predicted the US would fail as it had in Syria, Afghanistan and Iraq.

Israeli President Shimon Peres also sent a Nowruz message to Iran on Thursday, expressing his hope that the two countries could live in peace.

“I do believe that the young generation of Iran, like the young generation all over the world, will choose to live in peace, in cooperation. Instead of fighting, let’s compete in scientific endeavor. In competition everybody wins, in war everybody loses,” he said in a video message.

The Associated Press and AFP contributed to this report

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