Gaza City - Hamas threatened Tuesday to call off a five-month-old Gaza truce, after Israel kept up its near-total closure of the strip for the 14th consecutive day in response to ongoing rocket attacks.

A spokesman for the radical Islamist movement that rules Gaza called on all Palestinian militant factions active in Gaza to meet and reach a joint decision on whether to continue with the truce or break it off.

A small Israeli force also crossed into south-east Gaza Monday morning to comb the border area for bombs, sparking clashes with Palestinian gunmen, witnesses and the Israeli military said.

Meanwhile, three foreign activists from the United States, Scotland and Italy along with 14 Palestinian fishermen were taken in for questioning, after the Israel Navy intercepted their vessels which it said had ventured outside a fishing zone defined by the Israeli military off the shores of Gaza.

Israel has kept its border crossings with Gaza all but completely shut since the Egyptian-brokered truce, which took effect June 19 and is due to expire December 19, began crumbling two weeks ago.

Apart from a limited amount of industrial diesel for Gaza's only power plant brought in Thursday and 33 trucks of basic humanitarian supplies allowed in Monday, no goods or fuel have entered the Strip via Israeli border crossings since November 5.

The United Nations agency for Palestinian refugees (UNRWA) managed to resume its food distribution to residents of Gaza refugee camps Tuesday following Monday's aid shipment.

But the Gaza power plant, which supplies some 30 per cent of the strip's electricity, has stopped working as a result, leading to blackouts in Gaza City.

Witnesses said that three tanks and two bulldozers rolled 100 metres across the border east of the town of Rafah, in the southern Gaza Strip Tuesday morning.

An Israeli military spokeswoman said a small force was on a "routine activity" to uncover explosive devices, which are frequently planted by militants along the border fence.

She said local gunmen fired two mortar shells at the force.

Hamas' armed wing, the Izz al-Deen al-Qassam Brigades said in a statement that its fighters fired the projectiles.

The three foreign activists detained by Israel in the waters off Gaza were members of the pro-Palestinian International Solidarity Movement (ISM).

The Israeli military spokeswoman said they were taken in for questioning by the Israeli Immigration Police, while the fishermen were questioned by security forces.

She said their vessels sailed beyond a zone within which Palestinian fishermen are allowed to fish and ignored instructions to turn back.

The Gaza truce was thrown into uncertainty after an Israeli force crossed over the Strip's central border on November 4 to destroy a 250-metre-long tunnel dug by militants who Israel said were plotting to abduct soldiers from Israeli territory.

The force was confronted by local militants, sparking the heaviest clashes since the truce. Thirteen Palestinian militants, most of them of Hamas' armed wing, and one Palestinian civilian have been killed in those clashes and ensuing ones since then.

Gaza militants have fired more than 40 rockets and mortar shells into Israel. (dpa)