(Adds two militants killed, previously JERUSALEM)

By Nidal al-Mughrabi

GAZA, Feb 24 (Reuters) - Israel killed two Hamas militants on Sunday and vowed to prevent any attempt by Palestinians to break through the Gaza Strip's border with the Jewish state in defiance of an Israeli-led blockade.

A pro-Hamas group has said it will hold a peaceful protest in Gaza on Monday in which it has estimated that 40,000 to 50,000 women and children will form a "human chain" stretching the length of the Hamas Islamist-run Strip.

Hamas blew open Gaza's southern border wall with Egypt last month. Organisers of Monday's protest said they had no intention of breaching the border with Israel but an Israeli security source said the army was preparing for "all scenarios".

The breach of the Egyptian border at Rafah allowed hundreds of thousands of Palestinians to pour into Egypt to stock up on goods in short supply in the coastal enclave because of Israel's economic and military cordon.

Since then, Hamas officials have raised the possibility of organising similar breaches along the border with Israel.

"Israel will defend its territory and will stop any infiltration attempt into its sovereign borders," Israeli Defence Minister Ehud Barak and Foreign Minister Tzipi Livni said in a rare joint statement.

"Israel is acting to prevent an escalation but makes it clear that if it does happen the responsibility falls squarely on the shoulders of Hamas," the statement said. Ahead of Monday's protest, the Israeli army put its paramilitary border police on standby and boosted surveillance along the Gaza frontier, security sources said.

Hamas officials said two of its militants stationed near the border were killed in an Israeli air strike. Two others were wounded. The Israeli army confirmed the attack.

BREACH THE BORDER

Hamas official Sami Abu Zuhri stopped short of threatening to breach the border. "Our people will not allow the continuation of this situation regardless of the results, and we will work to break the siege by all possible means," he said.

"We hold them (Israeli leaders) responsible for any harm to the participants in these peaceful demonstrations".

Jamal al-Khudary, head of the Popular Anti-Siege Committee organising Monday's demonstration, said: "We do not have intentions of approaching the fence, either in the north or the south. We hope all the participants will abide by the instructions and we will try to prevent any violations."

Organisers said Hamas-controlled schools across Gaza would get time off to allow students to take part in the protest.

An Israeli security source said: "Obviously, if gunmen start shooting at the fence we will have to respond in kind and we are absolutely unwilling to countenance a situation where the fence is breached like it was at Rafah."

Israel's Channel Two television said the army had also deployed more artillery along the border with Gaza.

Israel stepped up its blockade of Gaza in June after Hamas routed Fatah forces loyal to secular President Mahmoud Abbas and seized control of the coastal territory, home to 1.5 million Palestinians.

Israel said it tightened its cordon last month, limiting supplies of fuel and other supplies, in response to cross-border rocket fire by militants in Gaza. (Additional reporting by Avida Landau and Dan Williams in Jerusalem; Writing by Adam Entous; Editing by Robert Woodward)

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