Israeli soldiers shot dead four Hamas gunmen in the Gaza Strip yesterday during an incursion in the Palestinian territory, drawing promises of revenge in a further setback to a fragile ceasefire.

Aid groups said they feared the flare-up could derail Israeli plans to reopen border crossings with the Gaza Strip today to allow in humanitarian supplies for the first time in over a week.

The UN Relief and Works Agency (UNRWA), which provides food and other essentials to Palestinians in the Gaza Strip, said Israel's blockade was targeting the world body directly.

"This has become a blockade against the UN itself," UNRWA spokesman Christopher Gunness said, citing a long list of supplies, including food, that Israel has yet to allow into the coastal territory.

An Israeli army spokesman said soldiers entered the Hamas-controlled Gaza Strip after opening fire at gunmen preparing to plant explosives along the border. The Islamist group said the men belonged to its armed wing.

Hamas spokesman Ayman Taha said: "This (Israeli) escalation is dangerous and we will not stand still. There will be a painful response against the Zionist enemy."

Palestinian militant groups in the territory launched eight mortar bombs into Israel during and after the clash, the army said. There were no casualties or damage.

A five-month-old ceasefire between Hamas and Israel along the Gaza frontier was disrupted last week by an Israeli army incursion and Palestinian rocket salvoes. Israel kept its border crossings with the Gaza Strip closed for a week after the violence.

It allowed some fuel shipments into the territory on Tuesday but food and other humanitarian supplies have yet to be transferred. UNRWA says it will be forced to suspend food distribution to 750,000 Gazans tonight unless supplies are allowed in.