GAZA CITY (CNN) -- Hamas security forces arrested 10 to 20 Fatah members in Gaza Thursday night and early Friday, adding to hundreds detained in the past week, Palestinian security sources said.

Fatah-affiliated senior officials were among those arrested overnight, the sources said.

Hamas spokesman Sami Abu Zuhri told CNN that the latest arrests were a response to the detention of Hamas members by Fatah in the West Bank overnight, including Hamas senior official Muhammad Ghazal.

A Palestinian source in the West Bank could not confirm any new arrest of Hamas members there but the source did say dozens arrested in recent days remained in custody. Others were released Thursday, the source said.

The arrests of Fatah members in Gaza and Hamas members in the West Bank began last Friday night after five Hamas militants and a child died when a bomb exploded on a Gaza beach.

Among the dead was Amar Musubah, a Hamas military commander, who has been the target of numerous assassination attempts by the Israeli military.

Fatah -- the party of Palestinian Authority President Mahmoud Abbas -- denied responsibility for the attack.

The two factions have been bitterly divided since Hamas militants drove Abbas' security forces out of Gaza last year.

Human Rights Watch released a report Wednesday concluding that infighting between the two main Palestinian factions has led to arbitrary arrests, torture and abuse of detainees by both sides.

Fatah and Hamas "have carried out a wave of unlawful arrests against opponents in recent days," and "Hamas forces physically abused some of the people they apprehended," the group said in its 113-page report.

Security forces on both sides act with impunity, the report said.

CNN's Ari Bell and Nidal Rafa and journalist Talal Abu-Rahmi contributed to this report.

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