By Nidal al-Mughrabi

GAZA, July 31 (Reuters) - The leader of the Islamist group Hamas, Ismail Haniyeh, threatened on Friday to boycott reconciliation talks with the rival Fatah group unless his supporters detained in the West Bank were released.

The two dominant Palestinian groups accuse each other of carrying out political arrests that have crippled Egyptian efforts to broker a deal to restore political unity and boost prospects for a resumption of peace-making with Israel.

"It is doubtful that this dialogue can succeed and it is doubtful that parties including Hamas would attend the coming round of dialogue in Cairo if we don't close the file on political arrests," Haniyeh said in a Friday sermon at a mosque in Rafah, in the southern Gaza Strip by the Egyptian border.

"We are not naive and we won't accept that dialogue takes place while arrests continue," Haniyeh said.

The next round of reconciliation talks is scheduled for Aug. 25 in Cairo. Egyptian mediators hope to get Hamas and Fatah to agree to some form of power-sharing ahead of presidential and parliamentary elections in January.

Hamas seized control of Gaza in 2007 after routing Fatah forces. Fatah is now dominant only in the West Bank. Haniyeh said his group would also boycott the January elections if the issue of political arrests remained unsolved .

Tensions have been rising over Hamas's decision this week to prevent Fatah lawmakers from leaving Gaza for the West Bank to attend their party's first congress in 20 years.

Fatah says some 300 of its supporters have been detained since Hamas seized control of the Israeli-blockaded Gaza. Hamas says some 1,000 of its supporters have been detained Fatah-dominated security forces in the West Bank.

Fatah officials on Friday said Hamas security men were seizing the passports and identification documents of Fatah lawmakers and activists to ensure they could not leave Gaza for the party congress, which Palestinian President Mahmoud Abbas is due to chair in Bethlehem on Aug. 4.

It is still not clear if the congress will go ahead. Fatah leaders expect to decide on Saturday whether to convene it without their Gaza-based peers, or to postpone once again.

A rift between the rival groups widened in 2006 after Hamas won a parliamentary election and opposed Abbas's peace talks with Israel. Abbas suspended peace talks seven month ago over Israel's Dec.-Jan. military offensive in the Gaza Strip. (Editing by Joseph Nasr and Philippa Fletcher)

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