The appointment of a divisive Gaza strongman to a top security post is threatening the new Palestinian unity government just days after its inauguration.

Mohammed Dahlan, a prominent member of the moderate Fatah Party, is despised by the rival Hamas group for leading a brutal crackdown on it in the 1990s. He also was one of the leading critics of Hamas during recent months of tensions and infighting between the two movements.

Now Dahlan is in charge of bringing order to the chaotic security services, including Hamas' own militia. Analysts say the issue is so sensitive that it could break up the new government and return Gaza to civil strife.

Dahlan, now a Fatah legislator, was appointed this week as security adviser to President Mahmoud Abbas of Fatah and to head up the National Security Council. The powerful body is to decide security policy and reform, making Dahlan more powerful than the interior minister, who only will have executive powers in security matters.

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The security council was formed as a part of a carefully cobbled agreement between political rivals Hamas and Fatah after months of tense negotiations to create a coalition government.

Those talks often deteriorated into deadly street battles between the group's militias and different government security apparatuses. The fighting killed more than 140 Palestinians from last May until a cease-fire was called in early February.

Experts said if the apparatuses aren't transformed into a disciplined national body, they'll quickly return to battling each other, as the coalition government runs into tension, which many fear is inevitable.

"If there's no reform of the security apparatus, it will be a threat to government's stability," said Hamdi Shakkour, of the Palestinian Center for Human Rights in Gaza, which documents abuses by security officials.

Nimr Hamad, a top aide to Abbas, defended Dahlan's appointment. "Everyone knows that Dahlan is an expert. It was a good choice," he said.

Dahlan also enjoys access to Israeli and U.S. officials.

Hamad says Hamas members "deal with him (Dahlan) in a very respectable way."

Still, many Hamas members accuse Dahlan of inciting and funding Fatah loyalists to battle Hamas during bouts of internal fighting. Hamas also claimed Dahlan tried to overthrow the previous Hamas-led government. Thousands of gunmen in Gaza are loyal to Dahlan.

There was little trust even before Hamas came to power in March 2006. In the 1990s, Dahlan headed a security apparatus in Gaza that cracked down on Hamas activists under the orders of the late Yasser Arafat. Many Hamas members say they were tortured by Dahlan's men.

In a sign of cracks in the new government, Hamas already has condemned Dahlan's appointment.

Khaled Mashaal, head of Hamas' political bureau, said in an interview to Arabic news channel Al-Jazeera that Dahlan was appointed without consultation.

"This issue should have been done in consultation, all appointments should have been done in agreement," Mashaal said.

Speaking to a Hamas Web site, Hamas lawmaker Salah Bardawil called Dahlan "undesirable."

"He has left his prints on ... the internal fighting," he said.

Powerful Hamas leader Mahmoud Zahar suggested the appointment was illegal, because Dahlan was a parliamentarian and could not take on executive roles.

Hamas and Fatah agree security reform is needed. There are 12 Palestinian security apparatuses, some controlled by the president, and others by the Interior Ministry.

The apparatuses have unclear, often overlapping roles, and there is poor discipline. Most security men are Fatah loyalists, and act according to factional loyalties and priorities.

In response, Hamas formed its "Executive Force" — a 5,600-strong militia in the Gaza Strip — when it came to power last year.

The two rivals disagree on what reform will mean. Hamas has called for unifying security apparatuses under the Interior Ministry. Fatah wants to dismantle the Executive Force. At a news conference this week, Dahlan played down the Hamas rivalry, saying the Executive Force "is not the biggest problem we face."

"Right now we have to work on other, more important issues," he told reporters, citing the wave of kidnappings, murders and other violence in Gaza.

Shaker Shabat, a Gazan analyst, said Dahlan is the only figure powerful enough to reform Palestinian security. "All the heads of the security apparatuses are loyal to Dahlan," he said.

Shabat said Dahlan would quietly move to dismantle the Executive Force by offering incentives to Hamas.

"Dahlan wont show Hamas that he's coming to increase Fatah's powers. He'll give Hamas some sweets, he'll appoint some of their men."

But he said any changes will ultimately favor Fatah. "The people with experience, with the military ranks, were recruited from Fatah years ago. Any developments will be internal, and they'll be Fatah loyalists," he said.