It is the first time Hamas has threatened to take action against Yasser Arafat's Palestinian Authority.

Sheikh Ahmed Yassin - Hamas leader under house arrest

The warning - in a leaflet faxed to western news agencies - comes after a renewed security operation by Palestinian police when the Hamas leader, Sheikh Ahmed Yassin, was placed under house arrest and hundreds of other activists were detained.

The statement by the military wing of Hamas, the Izz al-Din al-Qassam Battalions,accused the Palestinian Authority of betraying its own people.

BBC's Jeremy Bowen: "Instability always follows progress in peace talks"

It said "tough security measures against Hamas members may push some of them to aim their guns at Palestinian security men, instead of Israel".

Hamas used to insist that its only enemy was Israel.

Mainstream Hamas leaders disowned the threat. One, Ismail Abu-Shanab, told Qatari television that the statement "fully contradicts the Hamas strategy which seeks to spare the Palestinian society the evil of fighting regardless of circumstances or causes".

Showing support at a Hamas rally

Hamas is vehemently opposed to last month's interim Middle East peace agreement, under which Israel agreed to hand over 13% more of the West Bank in return for a concerted effort by the Palestinian Authority against Islamic militants.

BBC Jerusalem correspondent Jeremy Bowen says instability and danger always follow progress in the peace talks, and that threats by extremists should be taken seriously.

New settler protest

In a separate development, tensions rose in Jerusalem, when a hardline Jewish settler group started levelling a building site at Ras al-Amud, an overwhelmingly Palestinian part of the city.

Palestinian and Israeli peace campaigners chained themselves to the bulldozers in protest.

Abdel Bari Atwani: Consessions will reflect on Hamas

Meanwhile Israeli television has reported that Prime Minister Binyamin Netanyahu has approved the building of 200 homes at a Jewish settlement, Qiryat Arba, on the outskirts of Hebron.

The report said the work would start on Monday, and suggested it was a response to the killing of an Israeli guard last week by suspected Palestinian militants.