TEHRAN: Arab nations that normalised ties with Israel last year have "sinned" and should reverse such moves, Iran's supreme leader Ali Khamenei said on Sunday.

The United Arab Emirates, Bahrain, Sudan and Morocco all said in 2020 they would normalise ties with Israel, as Washington under the administration of then US president Donald Trump made Arab-Israeli rapprochement a foreign policy priority.

"Some governments have unfortunately made errors -- have made big errors and have sinned in normalising (their relations) with the usurping and oppressive Zionist regime," Khamenei said, referring to Israel.

"It is an act against Islamic unity, they must return from this path and make up for this big mistake," Khamenei added, in a speech marking a public holiday honouring the birth of the Prophet Muhammed (PBUH).

Iran has in the four decades since the 1979 Islamic revolution positioned itself as a strong defender of the Palestinian cause.

Israel 'Not A Country, But A Terrorist Base': Iran's Khamenei

Egypt and Jordan were until last year the only two Arab countries to normalise relations with Israel.

"If the unity of Muslims is achieved, the Palestinian question would definitely be resolved in the best fashion," Khamenei said.

In May, Khamenei characterised Israel as a "terrorist base" and "not a country".