Israeli forces moved into a densely populated Gaza refugee camp before dawn Tuesday morning, sealing off one neighborhood, while helicopter gunships fired missiles in two separate attacks, according to residents.

Soldiers went from house to house in the Tel Sultan neighborhood of the Rafah refugee camp in southern Gaza searching for militants and weapons, seizing the tallest buildings first. Meanwhile, bulldozers carved out a trench to seal off the area from the rest of the camp, witnesses said. They reported 12 Palestinians killed and 30 wounded. There were no immediate reports of Israeli casualties.

The first helicopter strike, at about 1 a.m., killed three people, two of them militants, and wounded 12, according to witnesses. A second missile strike at about dawn killed at least six people at the Belal mosque and set part of the building on fire, witnesses said. Three more people died in street fighting.

Army spokesman Capt. Jacob Dallal confirmed the two helicopter strikes and said they were aimed at groups of armed militants. He said the second strike was aimed at Palestinian gunmen in an area adjacent to the mosque and that the missiles had not been fired at the mosque itself. He also confirmed that soldiers were concentrating on the Tel Sultan area and had had frequent exchanges of gunfire with Palestinian gunmen there.

The incursion marked the start of what appeared to be the biggest operation in a Gaza population center since the Palestinian uprising began nearly four years ago. Soldiers backed by tanks, armored bulldozers and helicopter gunships had ringed the city on Monday, cutting off civilian traffic and firing at vehicles that attempted to skirt the blockade.

"This operation is aimed at the terrorist infrastructure in Rafah," Dallal said. "We're doing it more thoroughly than in the past. This is an in-depth operation for really curbing the flow of weapons by finding exactly the places and people and infrastructure that runs the smuggling operation."

Israeli officials had pledged to rid Rafah of fighters and weaponry and to destroy the extensive network of tunnels used to smuggle arms and ammunition across the border from Egypt into the Gaza Strip.

The operation followed three days of fierce fighting in the area last week in which seven soldiers and 19 Palestinians were killed and in which U.N. officials said 88 houses had been demolished and more than 1,000 refugees made homeless.

In response to an international outcry against the destruction of houses in Rafah, Israeli officials denied that the purpose of the new operation was to expand a 100- to 200-yard-wide strip of land known as the Philadelphi corridor that the army has carved along the Egyptian border to thwart smugglers. Any further demolition of houses would be the result of combat with Palestinian fighters, not a pre-authorized plan, an Israeli official said.

As troops massed outside the city and refugee camp, Palestinians inside laid tripwire booby traps and mines and took up ambush positions in anticipation of an Israeli attack, witnesses said.

Rafah residents with homes near the cleared swath of no-man's land continued to pack their possessions and evacuate the area, witnesses said. Some left voluntarily, while others fled after Israeli troops opened fire on Palestinians.

Abdel Halim Irdwan, who heads a new committee for residents of the Block O neighborhood, said six tanks moved into the area in the afternoon and opened fire. Two people were hurt and hundreds took flight, Irdwan said in a telephone interview. "We don't need any more martyrs," he said.

Ibrahim Shatat, an unemployed laborer who took up residence with his wife and eight children in the locker room of Rafah's soccer stadium after their home was bulldozed, said the several hundred people living there had grown panicky after hearing gunfire nearby, fearing an Israeli assault. He noted that the Palestinian Authority offices are nearby. "Many are afraid this is not a safe place," he said. "People have been taking tents and going to nearby groves, where it is considered safer."

Palestinian leaders, who have few options in staving off the military incursion, sought to rally international opinion. Prime Minister Ahmed Qureia, meeting in Berlin with President Bush's national security adviser, Condoleezza Rice, pleaded with the United States to intervene, a Palestinian official said. Rice told reporters afterward that house demolitions in Rafah were "a subject of concern" but went no further.

Frankel reported from Jerusalem. Researcher Samuel Sokol in Jerusalem contributed to this report.