A Palestinian militant has been seriously wounded after an Israeli aircraft launched a rocket at Hamas members in the southern Gaza Strip.
Israel is preparing to pull out of Gaza during the summer
The aircraft targeted a pair of Palestinian militants about to fire mortars at the Gush Katif Jewish settlement, the Israeli army said.
It is the first time Israel has fired missiles at Palestinian militants since a ceasefire was declared in February.
Overnight, a Hamas gunman was killed in disputed circumstances in Rafah.
Each side blames the other for the death of Ahmad Barhoum, who died on the edge of Rafah on the border between the Israeli-occupied Gaza Strip and Egypt.
Hamas said he was killed during a patrol when a grenade or some small explosive device was fired at him by Israel soldiers.
Israel says its forces only used automatic rifles in the exchange, so the explosives must have been deployed by the Palestinians themselves.
The Israeli air strike, combined with increased militant attacks on settlements, presents the toughest test to date for the truce, correspondents say.
Hamas appears to have been retaliating for the death of its member with mortar shells fired at Gush Katif, a settlement bloc which extends along the coast of southern Gaza.
The militants had fired at least four mortar shells and were reported to be preparing another barrage when the aircraft attacked.
An Israeli settler recieved minor injuries as a result of the mortar fire.
Hamas said one of its members was critically wounded while on a "holy mission".
Clashes in Gaza have been rare since the informal truce was declared in February, although sporadic mortar attacks have continued against settlements.
Settlers - who are to be evacuated under a unilateral Israeli disengagement plan in the summer - and their supporters have criticised the government's previous restraint.