GAZA - A four-month-old ceasefire between Israel and Palestinian militants in Gaza was in jeopardy yesterday after Israeli troops killed six Hamas gunmen in a raid into the territory.

Hamas responded by firing a volley of rockets into southern Israel, although no one was injured. The violence represented the most serious breach in a ceasefire agreed in mid-June. Despite the bloodshed, both sides suggested they wanted to return to a period of calm.

Israeli troops crossed into the Gaza Strip late on Tuesday night near the town of Deir al-Balah. The Israeli military said the target was a tunnel it claimed Hamas was planning to use to capture Israeli soldiers positioned on the border fence 250 metres (820ft) away. Four soldiers were injured in the raid, the Israeli military added.

One Hamas gunman was killed and Palestinians launched mortar bombs at the Israeli military. An Israeli air strike then killed five more Hamas fighters, including Wajed Moharib. Hamas launched 35 rockets into southern Israel, one reaching the city of Ashkelon.

"This was a pinpoint operation intended to prevent an immediate threat," the Israeli military said in a statement. "There is no intention to disrupt the ceasefire; rather the purpose of the operation was to remove an immediate and dangerous threat posted by the Hamas terror organisation."

In Gaza a Hamas spokesman, Fawzi Barhoum, said the Palestinian Islamist group had fired rockets as a "response to Israel's massive breach" of the truce.

"The Israelis began this tension and they must pay an expensive price," he added. "They cannot leave us drowning in blood while they sleep soundly in their beds."