Israel urged its citizens in Turkey on Monday to leave "as soon as possible" in an upgraded travel advisory predicting possible follow-up attacks to the March 19 suicide bombing in Istanbul blamed on IS group.

Three Israeli tourists and an Iranian were killed in the Istanbul attack, which prompted the counter-terrorism bureau in Prime Minister Benjamin Netanyahu's office to issue a generalised "level 3" warning against travel to Turkey.

A statement by the bureau raised this to "level 2" on Monday, signifying what it called a "high concrete threat" that IS group or similar groups would attack Turkish tourist attractions. It did not elaborate on what prompted the alert.

The statement said Israelis should avoid going to Turkey and, if already there, "depart as soon as possible".

If a "level 1" alert were by issued by Israel, that would urge citizens to leave the country "forthwith".