GAZA CITY, Gaza Strip – Israeli forces staged their deadliest raid into the Gaza Strip in more than a year Tuesday, killing a Hamas firebrand’s son and 17 other Palestinians and provoking threats by the group to escalate the conflict just as peace talks between Israel and the Fatah faction are starting.

The violence erupted days after President Bush visited Israel and the West Bank and underscored the fragility of the peace effort he is promoting in his visit to the Middle East.

Witnesses said about 20 Israeli tanks blasted into two Gaza City neighborhoods after Palestinian militants spotted an undercover army unit and pinned it down in a firefight. Israeli helicopters swooped in and launched three missiles, hitting a group of Hamas fighters in a car and two squads at separate mortar-launching sites.

The fighting broke out a day after Israel and Fatah began talks in Jerusalem on the most sensitive issues of the decades-old conflict.

U.S. and Israeli officials acknowledge that Hamas, the movement that controls Gaza and refuses to recognize the Jewish state, has the will and the weapons to undermine the negotiations, the first substantive Israeli-Palestinian peace talks since 2001.

Tuesday’s Israeli assault was aimed at pushing militants back from the border and dealt Hamas a sharp blow, killing 15 of its fighters. But Hamas seized on the timing of the raid to disparage the peace talks, and it warned of intensified fighting in the days ahead.

Standing over the body of his 21-year-old combatant son in Gaza City’s Shifa Hospital morgue, Mahmoud Zahar, Gaza’s most influential hard-line political leader, declared that Bush had given the Israeli assault a “green light” while visiting Jerusalem last week.

“Bush did not come to make peace but to encourage the Israelis to kill our people,” Zahar said, his voice shaking with emotion after kissing the bloodied forehead of his son, Hussam.

Expectations of a stepped-up Israeli offensive in Gaza had been building even before Bush’s visits to Jerusalem and Ramallah last week.

Israeli newspapers reported that Prime Minister Ehud Olmert and other Israeli officials briefed Bush on options for stopping the rocket fire from Gaza and concluded that Bush would not object to forceful measures as long as efforts were made to spare civilian lives.