GAZA, Feb 8 (Reuters) - Hamas's Gaza-based leadership challenged on Wednesday a Palestinian reconciliation deal signed by the Islamist group's political chief in exile and President Mahmoud Abbas of the rival Fatah movement.

Bringing divisions within Hamas to the surface, the group's "Change and Reform" Gaza parliamentary bloc came out against a key clause in the pact under which Abbas would serve both as president and prime minister of a future Palestinian government.

The legislative bloc includes Hamas's top Gaza-based leaders Ismail Haniyeh and Mahmoud al-Zahar. They did not attend the ceremony in Qatar where Hamas's political chief in exile, Khaled Meshaal, and Abbas signed the agreement on Monday.

Analysts have long spoken of a split within Hamas between those in the movement who have controlled the Israeli-blockaded Gaza Strip for the past five years and Meshaal, who had made his base in Damascus.

"We call upon the parties who signed and those who sponsored Palestinian reconciliation to reconsider and ... not to bypass Palestinian law," the parliamentary bloc said in a statement, arguing that a dual presidential-prime ministerial role for Abbas would be illegal.

The deal was aimed at reuniting the deeply split Palestinian national movement after past accords that followed Hamas's violent takeover of the Gaza Strip in 2007 failed to get off the ground over disagreements over who would head a new government.

SHUNNED

Hamas is shunned by the West over its refusal to recognise Israel, renounce violence and accept existing Israeli-Palestinian interim peace accords. A unity government seems likely to stop long-faltering Middle East peace efforts in their tracks.

Khalil Shaheen, a West Bank political analyst, said Gaza-based Hamas officials viewed acceptance of Abbas as prime minister as a political embarrassment, especially since Hamas defeated Abbas's Fatah in a Palestinian election in 2006.

He said Hamas could try to resolve its internal dispute by reorganising power-sharing between its leaders in exile and those in the Palestinian territories.

"Otherwise, and if the dispute continues, it could undermine the implementation of the agreement," Shaheen said.

Fatah lawmaker Abdallah Abdallah defended the Doha agreement saying there was no clause in the Palestinian law preventing Abbas from serving as both president and prime minister.

"It is clear that some people (in Hamas) have personal interests and not nationalist interests and they are trying to find a pretext to undermine such a step that paved the way towards ending the division," Abdallah told Reuters in the West Bank.

The accord is supposed to open the way for Palestinian presidential and parliamentary election possibly later this year, and to rebuild the Hamas-ruled Gaza Strip following a 2008-2009 Israeli offensive.