(Adds Israel to admit some fuel and aid)

GAZA, Nov 12 (Reuters) - Israeli soldiers shot dead four Hamas gunmen in the Gaza Strip on Wednesday during an incursion, drawing a volley of Palestinian mortar fire and promises of revenge in a further setback to a fraying ceasefire.

Tensions had already been running high over an Israeli halt to the import of fuel and humanitarian aid to Gaza following a similar clash on Nov 5. But Israeli Defence Minister Ehud Barak said some supplies would be allowed through on Thursday.

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

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

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

Aid groups said they feared the flare-up could keep Israel's more than week-old clampdown on Gaza in place. The impoverished coastal territory had already been under commercial embargoes and closures, stepped up after Hamas seized control last year.

"This has become a blockade against the United Nations itself," said Christopher Gunness, a spokesman for the U.N. agency for Palestinian refugees, UNRWA. He cited a long list of supplies, including food, that Israel was withholding from Gaza.

UNRWA said it would be forced to suspend food distribution to 750,000 Gazans on Thursday night unless supplies arrived.

In the evening, Barak's office said 30 truckloads of humanitarian aid would be allowed in to the territory on Thursday, as well as "a minimum allotment of fuel".

Last week's clash disrupted a five-month-old, Egyptian-brokered Gaza truce between Hamas and Israel.

Israel allowed some fuel shipments into Gaza on Tuesday.

Describing Wednesday's incident, Palestinian witnesses said Israeli forces entered a village near the southern Gaza town of Khan Younis and exchanged fire with the Hamas fighters. (Reporting by Nidal al-Mughrabi and Adam Entous; Writing by Joseph Nasr; Editing by Charles Dick)

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