GAZA CITY Ñ Hamas was said to have been dealt a harsh blow in wake of an Israeli air attack that killed 15 operatives.

Israeli and Palestinian sources said an Israeli missile strike on a Hamas training facility in Gaza City has again driven the Islamic insurgency group underground. The sources said the missile strike, directed by an unmanned air vehicle, on early Tuesday highlighted Israel's precise intelligence capability as well as the lack of Hamas security. The London-based A-Sharq Al Awsat daily said Israeli Air Force AH-64A Apache attack helicopters fired the U.S.-origin Hellfire missile toward the Hamas facility.

"The idea that Hamas could train outdoors in Gaza City undetected, regardless of the time of day, completely underestimated Israeli military capability," a Palestinian security source said. "If anybody could see this, then of course the Israelis would know about the training within minutes."

The sources said the Israeli attack marked the biggest blow to the organization since April 2004 when its leader,Abdul Aziz Rantisi, was killed in a helicopter strike. They said the death of 15 Hamas operatives and cadets would hurt recruitment and operations.

Hamas has been struggling since the death of Rantisi and his predecessor, Ahmed Yassin. Until last week, the organization was repeatedly foiled in efforts to carry out a massive suicide bombing inside Israel. On Aug. 31, 16 Israelis were killed in a double suicide bombing of Israeli buses in the southern city of Beersheba.

On Wednesday, an Israeli military unit captured a senior Hamas operative in the southern West Bank city of Hebron, where the Beersheba attack had been planned.

"We are interested in striking as many Hamas activists as possible whom we know to be in different stages of preparations for terror attacks inside Israel," Israeli Deputy Prime Minister Ehud Olmert said.

At the same time, Israeli combat units punched into the northern and central Gaza Strip in an effort to stop Kassam missile and mortar strikes on Israel. Hamas gunners also fired five Kassam missiles into Israel on Wednesday.

Israeli military sources said that over the last year Hamas has been adopting the tactics of the Iranian-sponsored Hizbullah. They said Hamas used a soccer field in the Sejaiyah neighborhood of Gaza City for training in the launch of Kassam-class short-range missiles, rocket-propelled grenades, mortars and explosives. But unlike the much larger area of southern Lebanon, where Hizbullah is based, Israel maintains easy access to most areas of the Gaza Strip.

"The targeted location was known as a gathering field of Hamas terrorists to be trained in the planting and activating of explosive devices, launching of RPGs and Kassam rockets and practicing infiltration into Israeli communities and military posts," an Israeli military statement said. "The training was led by senior Hamas terrorists who were involved in the carrying out of deadly terror attacks and attempted attacks."

The sources said that in late August, Hamas operatives assembled a large bomb in the soccer field, named after Yassin. They said the bomb, meant to be used in an attack against an Israeli target, prematurely exploded and a "well-known Hamas terrorist" was killed.

Hamas also used the soccer field to train in such tactics as hijacking an Israeli vehicle and abducting Israeli soldiers and civilians, the sources said. They said the field was used for youngsters at a Hamas camp in the daytime.

The Islamic insurgency group has pledged to retaliate for the Israeli missile strike. But Palestinian sources said a drop in funding as well as an inexperienced cadre of senior Hamas agents have hampered operations.

The Israeli strike has also sparked new tensions within the Palestinian Authority. Palestinian sources said PA Prime Minister Ahmed Qurei has again threatened to resign amid complaints that his authority was being usurped by PA Chairman Yasser Arafat.