GAZA, Oct 6 (Reuters/Nidal al-Mughrabi) - The armed wing of the Hamas group and its allies said on Wednesday they would retaliate against the Western-backed Palestinian Authority if it continued to take action against their members in the occupied West Bank.

The threat, made at a news conference in the Gaza Strip, underlined the depth of hostility between Hamas and the Palestinian Authority (PA) more than three years since the Islamist group seized control of Gaza in a brief civil war.

We say today that our silence will not last long. If those (security) services pursue their aggression, we will end the silence, said Abu Ubaida, spokesman of the Hamas armed wing in the Gaza Strip. He was flanked by gunmen from allied factions.

Abu Ubaida of Hamas's Izz El-Deen Al-Qassam Brigades was referring to a PA crackdown on Islamist activists including the sentencing this week of a Hamas fighter to 20 years in prison.

A PA spokesman said Alaa Abu Dhiyab, sentenced by a military court, was involved in the killing of three Palestinian officers in a shootout in the West Bank in 2009. Hamas says around 750 of its activists have been arrested by PA security forces since Hamas gunmen killed 4 Jewish settlers in the West Bank on Aug. 31 -- the eve of the launch of direct Middle East peace talks. Hamas opposes the US-backed talks.

Hamas accuses President Mahmoud Abbas's security forces of policing the West Bank on Israel's behalf. Abbas is a staunch supporter of peace negotiations aimed at creating a Palestinian state and is opposed to any violence against Israelis.

Abu Ubaida's threat came less than two weeks after Hamas and Abbas's Fatah faction agreed to revive efforts to narrow a schism between the two groups that has undermined the Palestinian cause.

If these reconciliation talks failed to ease the crackdown on Hamas, Abu Ubaida said no one should blame us if we pursue the symbols of the Fatah authority wherever they exist. Hamas leader Khaled Meshaal, who lives in exile in Damascus, met a senior Fatah official in Syria last month and further talks between the two sides are expected on Oct 20.

Accusations of political arrests by both sides are routine.

The Palestinian Authority, whose security forces are being retrained with US and European support, is determined to avoid any repeat of the Gaza takeover by Hamas.

The group, which defeated Fatah in a 2006 parliamentary election, is backed by Syria and Iran.

Last week, Palestinian Authority security forces also arrested a senior figure in the Islamic Jihad, a group allied to Hamas. Khader Adnan has been on a hunger strike for nearly a week in prison, Islamic Jihad said.