The Israeli military has launched an incursion into the Gaza Strip and clashed with Palestinian gunmen there.
Witnesses say the Israeli troops were accompanied by bulldozers which have been destroying agricultural land east of the town of Rafah, in southern Gaza.
There was no immediate reports of any injuries or deaths.
Meanwhile, Israeli defence officials say they have re-imposed a total closure of the border crossings into the Gaza Strip.
A convoy of 33 trucks of humanitarian aid was allowed in on Monday, but United Nations officials say the supplies will run out within days.
Israel says it has tightened its blockade of Gaza in response to recent rocket attacks by Palestinian militants, which have caused minor damage but no casualties.
Gaza map
A coalition of aid agencies which had been due to hold a meeting in Gaza held the event in the car park at Erez, the main crossing for people into Gaza, instead, in protest after they were denied entry to the Strip.
They called on the Israeli government to allow them "unimpeded humanitarian access" to help provide food, medical care and other services to Gaza's 1.5m residents.
The Israeli military described the incursion as "a routine operation to uncover explosive devices near the border fence in the southern Gaza Strip", the Associated Press news agency reported.
It said two mortars were fired at troops, causing no injuries or damage.
At least 16 Palestinian militants have been killed in skirmishes with Israeli forces along the border in the past two weeks.
Israel and the Palestinian Hamas group agreed a ceasefire last July, but it has been under serious strain since early in November.
Separately, in a separate incident, the Israeli navy detained a number of Palestinian fishermen and foreign peace activists who had been accompanying them in boats off the coast of Gaza.
The International Solidarity Movement said 14 fishermen and three activists were detained.
The Israeli military said the boats had deviated from the defined fishing zone, which the ISM denied.
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