"Israel has started a war with this assassination and it will pay a heavy price," he told AFP, speaking on condition of anonymity shortly after the two militants were killed in an Israeli missile attack on their car.

The two dead men were named as Fayez Abu al-Qaraa from the armed wing of Hamas and Hassan al-Madhun, a local commander of an armed wing of Al-Aqsa Martyrs Brigades, which is loosely affiliated to Palestinian leader Mahmud Abbas's Fatah party.

Qaraa, 38, was the first Hamas member killed in Israel's latest offensive on Palestinian militants in the Gaza Strip that was ordered after a deadly Palestinian suicide bombing in northern Israel last Wednesday.

The two were killed instantly when an Israeli aircraft fired a missile into their car, also injuring at least 10 civilians.

"There is no way we can speak about any quiet (period) faced with these continuous occupation crimes," Hamas's armed wing said, although it did not specify whether it was abandoning a de facto truce observed since late January.

Israel would be made to "repent of this crime", the group vowed in a statement.

"Our commander's blood weighs heavily on us and we will answer with blood for blood," it said. "You will see, not hear our response. The only language the enemy understands is war."

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