JERUSALEM — Gaza militants fired two rockets into southern Israel today, further straining a shaky, week-old truce as Israel kept vital Gaza border crossings closed in response.

The rocket attack, the second since the cease-fire took effect, led to a call for retaliation by Israel’s Foreign Minister while Palestinian Arabs charged that the continued closure of crossings violated terms of the cease-fire.

Despite the breach, 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 the Fatah party of the the president of the Palestinian Authority, Mahmoud Abbas, claimed responsibility for firing the rockets today. 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.

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.

Foreign Minister Tzipi Livni said it should not matter who was behind the rocket attack.

“I am not interested in 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.”

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

Since the cease-fire agreement took effect June 19, Israel has responded to rocket attacks by closing crossings rather than retaliating with airstrikes at Palestinian Arab rocket squads, as it did routinely since Hamas overran Gaza a year ago. Closure of the crossings, where vital supplies are shipped into Gaza, restores 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, Palestinian Arabs formed a committee to track Israeli violations.

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

At a meeting yesterday, 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 Arab 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, Prime Minister Salam Fayyad of the Palestinian Authority 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,” Mr. Fayyad said. “That is a situation that’s got to end.”

Hamas charged that the reimposed blockade is a violation, but a Hamas official, Taher Nunu, said the group remains committed to the truce.

“The (Hamas) government will not allow anyone to violate this agreement,” he said.

The rocket attack today came as an 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 Arab prisoners for an Israeli soldier Hamas has held 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 seaside territory. Israel has said it would not allow reopening of Rafah until the soldier is freed.