Gaza Strip ? An Israeli helicopter fired into the Gaza Strip on Saturday, killing a member of a militant squad that was clashing with Israeli forces in one of the most serious breaches of a 4-month-old cease-fire, Palestinian officials said.

The Israeli military said the air attack was aimed at militants planting a bomb along the border fence. It denied sending ground forces into the coastal territory.

But Palestinian officials who monitor the border said fighting erupted before dawn when plainclothes Israeli special forces slipped across the fence and were discovered by a squad of fighters from two small militant groups that refuse to honor the Nov. 26 truce.

The helicopter attack, apparently aimed at covering the special forces’ retreat, was part of a more aggressive Israeli policy that itself is straining the cease-fire. In the previous two weeks, Israel had staged an airstrike and a brief ground incursion against Palestinian gunmen, killing one in each operation.