By Nidal al-Mughrabi

GAZA, June 18 (Reuters) - A senior Fatah leader from Gaza, who sat out fighting with Hamas last week, called on Monday for an inquiry into how Palestinian President Mahmoud Abbas's forces lost control of the strip to the Islamist group.

The call from Ahmed Helles -- who had urged dialogue with Hamas instead of conflict -- was a sign of deepening political division between Gaza and the West Bank, where Abbas holds sway. Many loyalists of secular Fatah brand Helles a Hamas ally.

Helles was sidelined by Abbas earlier this year and is an arch rival of the president's national security adviser, Mohammad Dahlan, who is also from the Gaza Strip.

"A serious investigation must be conducted so that those who made mistakes can be held accountable," said Helles, who is respected by many of Fatah's armed groups. He said both Abbas and Dahlan should be included in any probe.

More than 100 Palestinians were killed in the fighting before Hamas routed the Fatah-dominated security forces.

"Helles did not fight and is trying to benefit from what happened in Gaza to impose himself as the leader," said one Abbas loyalist in the West Bank, reacting to the call for an investigation.

The takeover prompted Abbas to sack the Hamas-led Palestinian Authority government and appoint an emergency administration in the Israeli-occupied West Bank. Helles said Hamas had now closed all doors to dialogue with Fatah.

"Let the (Hamas-led) Authority collapse. Let the ministries collapse," said Helles. "This era will not last long."

Many Western officials and analysts believe Hamas made a pre-emptive strike before the United States and its Arab allies could build up Abbas's forces in Gaza. Hamas says it made its move in response to a U.S.-backed "coup".

Diplomats said poor planning by Fatah and Israel's delay of weapons shipments helped Hamas. The U.S. Congress held up funds and blocked training for many of Abbas's forces. Israel's public security minister, Avi Dichter, said Fatah's problems stemmed from poor leadership and command structure.

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