Hamas forces seized the leaders of Fatah in Gaza early Friday, Fatah officials said, upping the stakes in a week of tit-for-tat arrests between the bitter Palestinian rivals.

Hamas security officers seized around 15 senior Fatah members from their homes in the roundup, the Fatah officials said. They spoke on condition of anonymity because they fear for their safety in Hamas-controlled Gaza.

The men arrested Friday included three Fatah-affiliated district governors and the two highest Gaza representatives of the Palestinian president, Fatah's Mahmoud Abbas, the officials said.

Hamas spokesman Sami Abu Zuhri would not confirm the number or the identity of those arrested, but said the arrests were a response to Fatah's seizures of "Hamas political leaders" in the West Bank.

The two groups have engaged in a bitter and often violent rivalry for several years, culminating in Hamas' violent takeover of Gaza a year ago. Fatah's power in Gaza was largely erased and it was left in control only of the West Bank.

Both groups have periodically carried out arrest sweeps targeting rivals in the territories they control. Several of the men arrested by Hamas on Friday had already been seized in the past and later released.

The new Hamas crackdown was sparked by a mysterious July 25 explosion in Gaza that killed five Hamas members and a 6-year-old girl. Hamas blamed Fatah for the blast and began rounding up Fatah men.

Fatah denied the charge and launched its own retaliatory crackdown targeting Hamas men in the West Bank.

Fatah says more than 200 of its men have been seized over the past week. Hamas has refused to reveal how many arrests have been made. Fatah has arrested roughly the same number of Hamas men in the West Bank.

Abbas issued an order Thursday to free the detainees being held by Fatah, but it was not immediately clear if any had been released.