Iran's Supreme Leader Ayatollah Ali Khamenei said that the attempt of some regional states to normalise relations with Israel is "a great sin and mistake".
The governments, which recently established diplomatic relations with Israel, "must turn away from this anti-Islamic movement and make up for their great mistakes", Xinhua news agency quoted Khamenei as saying.
He urged for the unity among Muslim states to "restore Palestinians' rights" in the face of Israeli occupation of their lands.
"The main indicator for the unity of Muslims is the issue of Palestine, and the more seriousness is spent on the restoration of the rights of the Palestinians, the stronger the Islamic unity will become," Khamenei was quoted as saying.
He also expressed grievance over recent "unfortunate and tearful explosions" in Afghan mosques against Muslims and worshipers, saying that "they were done by the Islamic State militants, which the Americans explicitly stated that we created".
Also on Sunday, Secretary of Iran's Supreme National Security Council Ali Shamkhani played down Israel's recent threats and vowed his country's "tough" response.
He warned that Israel should think about rebuilding the damage caused by Iran's response, referring to Israel's allocation of substantial funds to counter Iran's nuclear program.
"Instead of allocating $1.5 billion budget for atrocities against Iran, the Zionist regime (of Israel) should focus on providing tens of thousands of billion dollars funding to repair the damage that is going to be caused by Iran's shocking response," Shamkhani tweeted.
Israel has allocated $1.5 billionto bolster its capabilities to attack Iran's nuclear facilities, the Western media reported.