The Sinai-based group, which claimed responsibility for a suicide bombing that killed two South Korean tourists and an Egyptian on Sunday, made the statement on an affiliated Twitter account.
"We recommend tourists to get out safely before the expiry of the deadline," read the tweet, written in English.
The warning has not appeared on the group's official website but the Twitter account has been accurate in the past.
Attacks by Islamist militants have killed hundreds of policemen and soldiers since the army deposed Islamist president Mohammed Morsi in July, but the bombing on Sunday of a tourist bus marks a strategic shift to soft targets that could devastate an economy already reeling from political turmoil.
Ansar Bayt al-Maqdis, Egypt's most active Islamist militant organisation, has threatened to topple the interim government installed by army chief Field Marshal Abdel Fattah al-Sisi.
The group's statement referred to a warning first issued on 16 February urging tourists to leave.