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JERUSALEM, July 3 (Reuters) - Gaza Strip militants fired a rocket into southern Israel on Thursday, prompting the Jewish state to order an early weekend closure of its border crossings with the Hamas-ruled territory, Israeli officials said.

There was no immediate claim of responsibility for the rocket attack, the sixth since an Egyptian-brokered truce between Israel and Hamas took effect on June 19.

An Israeli army spokesman said the rocket landed in an open field near the Gaza border town of Sderot and caused no casualties.

"Because of the rockets being fired today, the crossings will be closed tomorrow," said Peter Lerner, an Israeli defence official.

The crossings are generally closed from Friday afternoon to Sunday.

Israel has responded to previous rocket salvoes by closing border crossings used to bring supplies into the Gaza Strip, which Hamas Islamists seized from President Mahmoud Abbas's more secular Fatah forces a year ago.

The crossings were closed on Tuesday following a similar rocket attack. They reopened on Wednesday. (Reporting by Adam Entous; Editing by Mary Gabriel)

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