(Adds arrests)

By Wael al-Ahmed

JENIN, West Bank, May 19 (Reuters) - Palestinian forces loyal to President Mahmoud Abbas have arrested two Islamic Jihad leaders, a Palestinian security official said on Monday, in a U.S.-backed law-and-order campaign in the occupied West Bank.

Abdel Fatah Khuzaimiah, who was detained on Sunday, was the most senior militant taken into custody in the northern West Bank since hundreds of pro-Abbas security men deployed in the city of Jenin two weeks ago.

The security official said Jihad Nawadha, another Islamic Jihad commander in the area, was arrested by Palestinian forces on Friday.

Washington hopes the security campaign will show that Abbas can rein in militant groups -- Israel's main condition for Palestinian statehood. Both Khuzaimiah and Nawadha were long wanted by Israel.

In a separate incident, Israeli forces operating in Jenin early on Monday arrested Amar Saleh Abu Alon, a senior Islamic Jihad militant, an Israeli military spokesman said.

Islamic Jihad was responsible for numerous suicide bombings in Israel after a Palestinian uprising began in 2000.

Israel says its raids in the West Bank, condemned by Palestinian officials as undermining Abbas's security efforts, help to prevent attacks on Israelis.

In the Hamas-controlled Gaza Strip, Hamas security forces questioned Osama al-Fara, an Abbas loyalist and the governor of the southern town of Khan Younis, for four hours, sources in Abbas's Fatah movement said.

Ehab al-Ghsain, spokesman of the Hamas-run Interior Ministry, said Fara was asked about an invitation that was sent to Fatah supporters to attend at the governate building "the first celebration after the bloody Hamas coup".

Hamas seized the Gaza Strip from Fatah in fighting last June.

"We told (Fara) that governate headquarters belongs to all of the Palestinian people and must not be used by a single party to incite against others," Ghsain told Reuters.

Ghsain said the planned event was cancelled. Employees at Fara's office suspended work in protest at his interrogation.

Fara denied any Fatah involvement in organising the gathering.

"I believe that Hamas does not want like to see any institutions existing in Gaza except those that belong to it," Fara said.

Separately, Hamas forces released Bashir Abu Hattab, a senior employee at Abbas's office in Gaza who was arrested six days ago. The Interior Ministry spokesman declined immediate comment. (Additional reporting by Nidal al-Mughrabi in Gaza, Writing by Jeffrey Heller in Jerusalem; Editing by Alison Williams)

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