Military officials said initially the blast was apparently caused by ordnance left over from the 2006 Lebanon war, but later they also confirmed it was fired overnight.
"A 122mm Katyusha was fired overnight into northern Israel,'' said Avi Edri, a spokesman for the Israeli northern district police.
An Israeli military spokesperson said the blast in Shlomi village had caused no casualties, stressing that the investigation was ongoing.
Later, it was confirmed that two rockets were fired early in the morning hours.
The incident comes shortly before U.S. President George Bush starts a six-day tour of the Middle East, due to arrive in Israel on Wednesday.