Israel said its troops had uncovered a tunnel along central Gaza's frontier which had been dug by militants intending to abduct Israeli soldiers.

Clashes ensued when troops were sent to thwart the threat, Israel said. One militant died, Palestinian reports say.

A subsequent Israeli air strike on Hamas positions in southern Gaza killed at least five fighters, medics said.

An Israeli army spokeswoman said the air strike targeted militants who had fired mortars at Israeli forces.

Tuesday evening's fighting broke out after Israeli tanks and a bulldozer moved 250m into the central part of the coastal enclave, backed by military aircraft, says the BBC's Aleem Maqbool in Ram allah. Residents of central Gaza's el-Bureij refugee camp said a missile

fired from an unmanned Israeli drone flying over the area injured another three Hamas gunmen.

Rights violation

A truce between the two sides had held since it was declared on 19 June.

Israel said the raid was not a violation of the ceasefire, but rather a legitimate step to remove an immediate threat.

Mary Robinson, file pic from August 2008

Gaza residents 'terribly trapped'

But the militant wing of Hamas said it would take revenge for what it said was an act of Israeli aggression that had violated the truce.

"Our response will be harsh, and the enemy will play a heavy

price," the Islamist group said in a statement on its website.

Israel tightened a blockade on Gaza after Hamas took control there in June 2007.

Israel said the blockade, under which it has allowed little more than basic humanitarian aid into Gaza, was needed to isolate the militant group and stop it and other militants from firing rockets into Israel.

But a former UN High Commissioner for Human Rights said earlier she had been taken aback by the "terrible" conditions for the Gaza Strip's 1.4m residents during a recent visit.

Mary Robinson told the BBC it was "almost unbelievable" the world did not care about what she called "a shocking violation of so many human rights"