By Andrew Mills, Jana Choukeir and Ahmed Elimam

DOHA/DUBAI (Reuters) -Israel launched an airstrike against Hamas leaders in Qatar on Tuesday, expanding military actions that have ranged across the Middle East to include the Gulf Arab state where the Palestinian Islamist group has long had its political base.

Qatar, which has acted as a mediator alongside Egypt in talks on a ceasefire in the almost two-year-old war in Gaza, condemned the attack as "cowardly" and called it a flagrant violation of international law.

Two Hamas sources told Reuters that Hamas officials in the ceasefire negotiating team survived the attack, which followed an evacuation order in Gaza City where Israel is waging an offensive to try to destroy the group and its military capabilities in the Gaza Strip.

Israeli officials told Reuters the strike was aimed at top Hamas leaders including Khalil al-Hayya, its exiled Gaza chief and top negotiator. In Washington, a White House official said Israel had notified the U.S. about the strike beforehand.

The attack took place shortly after Hamas' armed wing, the al-Qassam Brigades, claimed responsibility for a shooting that killed six people at a bus stop on the outskirts of Jerusalem on Monday.

The assault is likely to deal a serious, if not fatal, blow to........

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