Palestinian militants in the Gaza Strip fired a makeshift rocket into southern Israel on Thursday, putting further strain on a week-old truce as Israel kept border crossings into the Hamas-ruled territory closed.

Both sides traded blame for violations of the ceasefire Egypt had brokered in the hope of furthering U.S.-backed peace talks that have shown little sign of progress.

Prime Minister Ehud Olmert came under pressure from a strong rival, Foreign Minister Tzipi Livni, to act. Livni, Israel's chief negotiator with the Palestinians, said she urged him to order "an immediate military response to every violation".

No casualties were reported in the rocket attack, which was claimed by al-Aqsa Martyrs Brigades, a militant group belonging to Palestinian President Mahmoud Abbas's Fatah faction.

The latest strike followed salvoes on Tuesday launched by Islamic Jihad in response to an Israeli army raid that killed one of the group's commanders in the occupied West Bank. Israel said the raid was aimed at foiling attacks on its citizens.

The rockets prompted Israel to keep Gaza crossings closed for a second consecutive day. Under the ceasefire, the flow of goods into the territory is meant to rise gradually.

The truce, which took effect last Thursday and is known locally as the "calm", does not cover the West Bank, but several militant groups had threatened to retaliate for any Israeli operations there.

"The rocket attack was in response to Israeli violations. Any calm deal must end Israeli attacks on our people in the West Bank too," said Abu Qusai, spokesman for al-Aqsa Martyrs Brigades.

Hamas said it would "take the necessary measures" against other factions to ensure the ceasefire is held.

Abu Qusai rejected the Hamas threat.

U.N. sources said that since the truce started, Israeli forces fired into the Gaza Strip at least eight times, wounding two people. Palestinians fired rockets and mortars bombs into Israel at least three times, according to the Israeli army.

CROSSINGS CLOSED

Israeli military liaison official Peter Lerner told Reuters that Israel's crossings with the Gaza Strip would remain closed on Thursday and no date had been set for their reopening.

"It depends on the assessment of the situation following Tuesday's rocket attack," Lerner said.

Raed Fattouh, the Palestinian coordinator of supplies into Gaza, said he was told by Israel that at least one of the border crossings would be opened on Friday.

Israel wants the opening of the Gaza-Egypt border to be conditional on a deal with Hamas for the release of an Israeli soldier, Gilad Shalit, seized by militants two years ago.

An Israeli negotiator planned to hold talks in Cairo later on Thursday with Egyptian intelligence chief Omar Suleiman as part of Egypt's efforts to broker a deal to exchange Shalit for Palestinians held in Israeli jails.

Israeli officials said the negotiator would propose including as many Gazans as possible among prisoners who would go free, in a bid to avoid a mass release in the West Bank that would strengthen Hamas in the territory at Abbas' expense.

Israel, the officials said, also wanted the right to determine whether prisoners who resided in the West Bank would return to the territory or be sent to the Gaza Strip on release.

Osama al-Muzaini, a senior Hamas official familiar with the Shalit talks, said the group was sticking to a list of prisoners that Egypt handed to Israel on its behalf.

"The talks have been frozen for 11 months because Hamas is not willing to make any changes to the names," Muzaini said.

Hamas spokesman Sami Abu Zuhri accused Israel of breaching the ceasefire and said a continued blockade would make it difficult for other factions to abide by the truce.

Israel sharply cut back on the supply of goods into the Gaza Strip a year ago, after Hamas took over the coastal enclave from forces loyal to Abbas's more secular Fatah faction.