Gaza militants fired two rockets into southern Israel Thursday, causing no injuries but further straining a shaky, week-old truce. Israel did not hit back, but it kept vital Gaza crossings closed, triggering Palestinian charges that Israel was violating terms of the cease-fire.

Despite the rocket attack, the second since the cease-fire took effect, Israel dispatched an envoy to Egypt in hopes of negotiating a prisoner swap with Gaza's ruling Islamic Hamas.

The Al Aqsa Martyrs Brigades, a violent offshoot of moderate Palestinian President Mahmoud Abbas' Fatah, claimed responsibility for firing the rockets. One exploded harmlessly in a field, the military said, refusing to disclose where the other landed. A statement from the militant group demanded that Israel halt its military operations in the West Bank.

Israeli Foreign Minister Tzipi Livni said Israel should retaliate. "I am not interested who fired and who didn't fire at Israel," she told reporters. "It is a violation, and Israel needs to respond immediately, militarily, for every violation."

The truce, hammered out by Egypt over months of separate talks with Israel and Hamas, does not include the West Bank. On Tuesday, Islamic Jihad fired three rockets at Israel, wounding two people, linking the attack to an Israeli raid in the West Bank.

Previous truces have come apart quickly because Gaza militants claimed the right to retaliate for Israeli West Bank raids.

Instead of retaliating for the rocket attacks with air strikes at Palestinian rocket squads, as it did routinely during the year since Hamas overran Gaza a year ago, Israel closed crossings where vital supplies are shipped into Gaza, restoring a blockade that has caused severe shortages.

That hits at the main interest of Hamas — ending the blockade and easing the hardships facing the people under its control. Hamas officials charged that by restoring the blockade, Israel is violating the truce. Underlining the high level of distrust, Palestinians formed a committee to track Israeli violations.

Israel was expected to keep the crossings closed Friday because of the latest rocket attack.

At a meeting Wednesday, Israeli defense officials discussed how to proceed once the crossings are reopened. According to officials who spoke on condition of anonymity because the meeting was closed, Israel might reset the truce clock each time it closes the crossings in response to a Palestinian violation.

Israel had significantly increased the amount of supplies flowing into Gaza on Sunday, in accordance with the truce agreement, and was ready for another increase next Sunday. But the rocket attack stopped the process. Now Israel is considering counting three days from each reopening of the crossings before it reinstates the original increase.

During a visit to Prague, Czech Republic, Palestinian Prime Minister Salam Fayyad said Israel should reopen the crossings to preserve the truce.

"(The reopening is) important because the closure ... of Gaza is actually producing a situation where you have 1.5 million of our people who live there with a sense of not much to lose," Fayyad said. "That is a situation that's got to end."

Hamas charged that the reimposed blockade is a violation. But Hamas official Taher Nunu that Hamas is committed to the truce. "The (Hamas) government will not allow anyone to violate this agreement," he said.

The rocket attack Thursday came as Israeli envoy Ofer Dekel headed to Egypt to meet with Egyptian officials on the final stage of the truce — a swap of hundreds of Palestinian prisoners for an Israeli soldier Hamas has held captive for two years. Israel has balked at Hamas' demands, saying its list of prisoners includes militants involved in deadly attacks on Israelis.

Hamas also has demanded that Israel allow reopening of Gaza's only border crossing with Egypt in the final phase of the six-month truce deal.

The Rafah crossing has been sealed since the Hamas takeover, confining Gaza's people to the tiny seaside territory. Israel has said it would not allow reopening of Rafah until the soldier is freed.