JERUSALEM, April 23 (Xinhua) -- Israel on Tuesday warned Israelis vacationing in Egypt's Sinai Peninsula to leave "immediately," citing threats of terror attacks.

The official warning was released by Israel's Counter-Terrorism Bureau hours after the Islamic State (IS) militant group claimed responsibility for the recent bombing attacks in Sri Lanka, which claimed the lives of at least 321 people and injured hundreds of others.

The Counter-Terrorism Bureau, operating under the Israeli Prime Minister's office, said in a statement that it is the Level 1 warning, the highest level of warning which is related to a "very high and concrete threat."

"The Counter-Terrorism Bureau calls on all Israelis (in Sinai) to leave the area immediately and return to Israel," the bureau said.

The bureau also asked Israelis to contact their relatives in the Sinai Peninsula about the warning.

Sinai Peninsula is the stronghold of terrorist groups in Egypt, the strongest of which is Sinai State, an Egyptian branch of the IS.

According to the estimates of the Israel Airports Authority, some 40,000 Israelis have travelled to Sinai's coastal resorts to spend the week-long Jewish Passover holiday which started on April 19, and 45,000 more are expected to go there in the next few days.