Hamas and Islamic Jihad made the warning in a joint statement to The Associated Press in Lebanon and accused the Palestinian Authority of bowing to Israeli demands it disarm the militants — a step Israel says is necessary for the 2-week-old lull in violence to hold.
Although Palestinian Prime Minister Mahmoud Abbas says he will not order security forces to disarm the militants for fear of civil war, police seized some weapons this weekend in the Gaza Strip, a Palestinian security source said, in what appeared to be a first effort to comply with the demand.
The tensions came as Israeli Prime Minister Ariel Sharon traveled to Europe for meetings on the U.S.-backed "road map" peace plan, and as Israeli and Palestinian security forces searched for an Israeli taxi driver feared kidnapped by Palestinian militants aiming for a prisoner swap with Israel.
The issue of disarming the militants is deadlocking the peace plan, which aims to end 33 months of violence and establish a Palestinian state by 2005. The plan requires the Palestinians to dismantle militant groups whose attacks killed hundreds of Israelis during the fighting.
Israel pulled troops out of parts of Gaza and the West Bank town of Bethlehem last week — a move also required by the road map. But Israel refuses to hand over more towns until the Palestinian Authority disarms the militants. Israeli officials have said the militants are using the break in the fighting to regroup and rearm.
Israeli officials have warned that unless they see action by the Palestinian Authority soon, Israel may take action itself.
In the past few days, Palestinian security forces have been confiscating illegal weapons and arresting people possessing unlicensed guns in Gaza, a Palestinian security source said. The source denied reports that Hamas and Islamic Jihad were specifically targeted and said arms were confiscated "to show that people will not be allowed to roam the streets with illegal weapons."
Israeli Defense Minister Shaul Mofaz also told Israel TV the Palestinians have arrested about 20 militants during the past few days. He did not say which groups they belonged to.
Hamas and Islamic Jihad accused the Palestinians of giving in to Israeli and U.S. pressure.
Palestinian security forces "have started a campaign that aims to disarm the Palestinian resistance in Gaza, which is a step that represents ... a red line which we will not allow to be crossed under any circumstances," their statement said.
They warned that attempts to disarm the militants "might lead us to think seriously about going back on our initiative to suspend military operations."
In Gaza, Hamas leader Abdel Aziz Rantisi told the AP, "Our initiative to temporarily stop our operations does not mean that we will give up our weapons."
The two groups called a three-month truce on June 29, and the Al Aqsa Martyrs' Brigades, a militant group affiliated with Yasser Arafat's Fatah movement, declared a six-month halt in attacks.
Hamas and Islamic Jihad have strong followings in the West Bank and Gaza Strip, and the Palestinian Authority fears that fighting the militants could tear apart Palestinian society.
Also Sunday, Palestinian security forces were helping Israeli police and soldiers hunt for 61-year-old Tel Aviv taxi driver Eliyahu Goral, who disappeared Friday.
"We assume that this was a kidnapping by a terror organization," Mofaz said.
Mofaz added that he spoke with Palestinian security chief Mohammed Dahlan and "I made it clear to him that the responsibility for (Goral's) well-being lies with the Palestinians."
Sharon arrived in London on Sunday for talks aimed at repairing strained relations with Britain and moving the peace process forward. He will meet with Prime Minister Tony Blair and Foreign Secretary Jack Straw.
Peace moves have been stalled not only over the issue of disarming the militants but also over the Palestinian insistence that Israel release the estimated 7,000 prisoners it holds. Israel has agreed to free only several hundred.
British-Israeli relations have been strained since Britain hosted a January conference on Palestinian reform and did not invite Israel. Israel prevented Palestinian delegates from traveling to the meeting.
Blair was expected to discuss with Sharon such issues as illegal Jewish settlement outposts in the West Bank, a British official said. Sharon is expected to urge Blair to cut ties with Arafat.
Also Sunday, Abbas met with Russian Foreign Minister Igor Ivanov to discuss peace moves.
Sharon is also expected to visit Washington later this month. Abbas is also expected to be invited, although his Fatah Party is pressuring him to refuse such a visit until Israel restores freedom of movement to Arafat.