Extremists on both the Israeli and Palestinian sides pose a significant threat to a truce that 13 Palestinian factions agreed to extend this week.

Rogue Palestinian groups, some of which are believed to be funded by the Lebanon-based Hezbollah (search), remain outside the truce, which is underpinning the new momentum toward peacemaking that has brought a recent drop in violence.

And Jewish extremists could upset the deal if they make good on threats to stage provocative demonstrations aimed at derailing Israel's planned pullout from the Gaza Strip.

The fragility of the truce was illustrated Friday by a statement from the Popular Resistance Committees (search), which said it would not abide by the cease-fire declared in Cairo on Thursday because it was not invited to the talks.

The small, Gaza-based group is made up of Islamic militants who defected from other groups and use tactics similar to Hezbollah guerrillas. It is responsible for blowing up three Israeli tanks and killing seven soldiers in three attacks in 2002 and 2003.

Mahmoud Abbas (search), the Palestinian Authority president, scored a diplomatic success in getting the main Palestinian factions, including Hamas and Islamic Jihad, to support the peace efforts by agreeing to extend a "period of calm" until the end of the year. An informal truce was declared by Abbas and Israeli Prime Minister Ariel Sharon at a Mideast summit Feb. 8.

Participants in the Cairo talks warned the deal could collapse if Israel does not hold its own fire and release all 8,000 Palestinian prisoners it holds — something the Jewish state has ruled out.

Palestinian officials have repeatedly cited Hezbollah as the biggest threat to the truce, saying it is offering money to local militants who stage attacks.

Israel also has expressed concern about Hezbollah, which is both a political party and a well-armed military force. Foreign Minister Silvan Shalom said recently that Hezbollah operates dozens of militant cells in the West Bank, works closely with violent Palestinian groups and offers them millions of dollars in aid.

A senior Israeli military official recently said 70 percent of the Al Aqsa Martyrs' Brigades (search) cells in the West Bank get Hezbollah funding.

There has been a sharp drop in fighting since the informal truce was declared in February, down from about 50 or more deaths a month to one so far in March.

"We are at the moment enjoying a period of calm," Israel's military chief, Lt. Gen. Moshe Yaalon, said Friday. "Is this a temporary respite or shall we be able to look back and say this is the end of the round of violence that started in September 2000. That question is still open."

Auguring well for the cease-fire is widespread Palestinian fatigue from the violence and moves by militants to join the political process.

A member of the Al Aqsa Martyrs' Brigades, which is has loose ties with Abbas' Fatah political faction, said Friday that Hezbollah guerrillas have been pressuring him to keep fighting Israel.

"Two weeks ago I received a phone call from a Hezbollah operator who told me that he is planning an attack and wanted me to help," the militant told The Associated Press on condition of anonymity.

The West Bank militant said he refused because "we have a chance to stop the fighting and let our people resume their normal lives."

A Palestinian security official in the West Bank confirmed that Hezbollah is trying to stir up violence but said officials are working hard to persuade militants to resist the pressure.

"Now we have orders to arrest anyone who is planning to carry out an attack," the official said.

Abu Mojahaid, an Al Aqsa leader in the West Bank, said renewed cooperation between Israeli and Palestinian security forces in the West Bank has complicated matters for Hezbollah.

"Now on a daily basis Israeli security provides Palestinian security with information, including recorded phone calls between Hezbollah militants, Hezbollah officers and al Aqsa militants," Mojahaid said.

Hezbollah officials were not available for comment Friday. But the group said last month that it would not try to disrupt the Israeli-Palestinian truce.

Both Hamas and Islamic Jihad are seeking roles in the Palestine Liberation Organization, which is dominated by Fatah.

"Hamas is getting into the Palestinian political regime now and it will not resort to violence," said Ali Jarbawi, a professor of political science at Bir Zeit University in the West Bank.

Although Israel rejects any negotiations with the militants, Sharon hailed the truce declaration as a "positive first step." While continuing to demand that Abbas disarm the militants, Israel has pledged to refrain from military activity as long as the situation remains quiet.

Jewish extremists, however, still could disrupt the calm. This threat was highlighted by footage aired Wednesday on Israel's Channel Two television showing a meeting of extremists, including rabbis, discussing plans to send thousands of Jews to a disputed holy site in Jerusalem sacred to both Muslims and Jews.

The purpose of this act, which would likely provoke a Palestinian outcry, would be to draw the army and police away from evicting settlers from Gaza and the northern West Bank this summer.

Israeli security services have repeatedly said they fear violence by a small number of Jewish extremists who vehemently oppose Israeli withdrawal. Settler leaders, however, have urged their supporters to use only peaceful means of resistance.