Israel is on high alert for possible attacks by Palestinian militant groups who are vowing revenge for Israeli raids in the Gaza Strip Wednesday in which 15 Palestinians were killed and scores more were injured.

Israel's Army Radio says troops have been put on high alert throughout the country in anticipation of possible revenge attacks by Palestinian militants.

The militant group, Hamas is warning of bloody attacks to come in retaliation for Israeli military operations in the Gaza Strip on Wednesday.

One Hamas leader told Israel Radio the group had sent out the word to its cells in Gaza and the West Bank to prepare to strike at Israelis everywhere.

Hamas spokesman Abdelaziz Rantisi said Israelis would pay a heavy price.

Israeli forces clashed with armed militants in two separate raids Wednesday in what turned into the bloodiest day of operations in recent months. In eastern Gaza City, 12 Palestinians were killed, while another three were killed in Rafah, near the Egyptian border.

The military says the raids were designed to crack down on militant cells preparing to launch attacks against Israeli targets.

Prime Minister Ariel Sharon has said if peace talks with the Palestinians fail, he is prepared to disengage unilaterally by removing almost all the Israeli settlements from Gaza.

Some analysts say Israel will want to show toughness prior to any pullout to make sure Palestinian militants do not see such action as a victory. Some see Wednesday's operations in Gaza as part of a muscle flexing exercise.