Tel Aviv - Israeli troops left the Gaza Strip early Wednesday, hours after entering it in the most serious clash between the Israel Defence Force (IDF) and Palestinian militants since a ceasefire went into effect June 19.

The raid left six Palestinians - five militants and a civilian woman - dead, medical officials said.

A military spokeswoman in Tel Aviv said the troops went into the strip late Tuesday to destroy a 250-metre-long tunnel being dug under the border, which, she said, was being prepared to aid in the kidnapping of Israeli soldiers.

The operation was a "pinpoint" one, she said, to destroy an "immediate threat."

Militants from the Islamic Hamas organization traded fire with the soldiers and the movement's armed wing said it had also fired at least 10 mortar shells at the Israeli force.

At one stage, an Israeli aircraft also targeted the Hamas militants.

Hamas' armed wing said it would "confront any aggression against our people and our militants," and accused Israel of violating the ceasefire.

The IDF, for its part, said in a statement Wednesday morning that the construction of the tunnel close to the border and aimed at facilitating the abduction of Israeli soldiers, was "a blatant violation of the ceasefire."

The IDF had had no intention of disrupting the truce, the statement added.

Gazan militants responded to the Israeli operation by launching at least 35 missiles toward southern Israel overnight and Wednesday morning, the military spokeswoman said. (dpa)