(Adds Meshaal comments)

By Nidal al-Mughrabi

GAZA, April 28 (Reuters) - Israeli soldiers shot dead three Hamas militants and critically wounded a fourth near Gaza's border fence with Israel on Saturday in what the army said was a thwarted attack, casting fresh doubt on a shaky ceasefire.

Hamas spokesman Fawzi Barhoum said the Islamist militant group, which leads a Palestinian unity government, has the right to respond to the deaths by "all means available".

He called the incident a violation of the five-month-old Gaza ceasefire, which all but collapsed earlier this week when Hamas's armed wing started firing rockets into Israel.

An Israeli army spokeswoman confirmed the incident near the border fence in central Gaza that killed the three Hamas militants.

The spokeswoman said the soldiers had opened fire on four Palestinian militants as they were placing explosive devices.

Abu Ubaida, a spokesman for the Hamas armed wing, said the four militants were on a "jihadist mission when the Zionist enemy opened fire on them".

"The occupation has no interest in calm and from our side we say there is no calm for free. The Zionist enemy will be punished for their crimes," Abu Ubaida said.

In a separate incident in southern Gaza, a Palestinian civilian was killed on Friday night by what local medics and residents said was an Israeli tank shell. The Israeli army said it was not involved.

CEASEFIRE The armed wing of the ruling Hamas movement broke the ceasefire earlier this week by firing rockets into Israel in response to the killing of nine Palestinians by Israeli forces last weekend.

Hamas later agreed to resume the ceasefire, halting rocket attacks, though other groups have not completely stopped the attacks.

"This is the right of Palestine; it has the right to defend itself, and there were nine martyrs in one day, and violations that must be responded to," Hamas leader Khaled Meshaal told reporters in Cairo.

Israeli Prime Minister Ehud Olmert decided in talks with security chiefs on Wednesday to step up "targeted attacks" against Palestinian rocket-launching crews, but he ruled out for now a major incursion into the coastal strip.

Palestinian Prime Minister Ismail Haniyeh of Hamas said the group would reassess its strategy in one to two months if international sanctions against the Palestinian government were not lifted.

Western powers imposed sanctions on the Hamas-led Palestinian Authority a year ago to pressure the militant group to recognise Israel, renounce violence and abide by interim peace deals.

"The reality on the ground in Palestine is the continuation of the embargo on the Palestinian people ... and Arab steps to lift the embargo are very slow," Meshaal said.

Haniyeh leads a unity government with President Mahmoud Abbas's Fatah faction. (Additional reporting by Aziz El-Kaissouni in Cairo)

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