Israel has kept three of its border crossings with the Gaza Strip closed for a second day, as an Israeli official travelled to Egypt to negotiate the release of Gilat Shalit, an Israeli soldier held by Palestinian fighters.

The crossings were closed on Wednesday after a number of rockets were fired across the border, in what Israel said was a violation of a six-month ceasefire agreed with Hamas.

Islamic Jihad, which claimed responsibility for the attack, said it was retaliation for the Israeli army's killing of one of its commanders in the occupied West Bank.

The truce between Israel and Hamas entailed a gradual easing of a blockade Israel imposes on the Palestinian territory as well as an end to attacks by both sides.

The truce, brokered by Egypt, does not cover the West Bank.

"The Gaza crossings are still closed," Peter Lerner, an Israeli military spokesperson, said on Thursday.

The Erez passenger terminal remained open for humanitarian purposes, he said.

Lerner said that the new restrictions were imposed "in response to the firing of rockets on Tuesday".

Asked when supplies would again be allowed into Gaza, he said: "This will depend on the security situation - there is an ongoing evaluation."

Islamic Jihad vow

After the crossings were shut on Wednesday, Islamic Jihad said that it would abide by the truce but warned it would respond if Israel was seen to have violated the agreement.

"We have confirmed to our friends in Hamas that we have decided to respect the ceasefire," Daoud Shihab, a spokesperson for the group, told the AFP news agency:

"We will apply the pact on the suspension of attacks if Israel also respects it," he said.

He said that a "committee comprising representatives of Jihad, the Popular Front for the Liberation of Palestine, the Democratic Front for the Liberation of Palestine and Fatah will meet when Israel breaches the truce to determine a riposte".

Israel imposed the blockade after Hamas took power of the Gaza Strip in June 2007.

The World Bank says the embargo has crippled the Gazan economy, where about 80 per cent  but Israel says it has allowed enough supplies to be transported to the strip.

Shalit negotiations

Meanwhile, Ofer Dekel, an Israeli envoy, is due in Cairo on Thursday to press Egyptian authorities not to reopen the Rafah border crossing until Corporal Gilad Shalit, who was captured two years ago, is released.

Rafah, the only Gaza crossing that bypasses Israel, has been closed since 2006, although Egypt has allowed some medical patients to cross the border.

Dekel is to meet Omar Suleiman, an Egyptian intelligence chief, who played a key role in mediating the truce which came into effect on June 19.

Hamas said on Wednesday that Shalit's release is unrelated to the truce agreement and that he will only be freed in exchange for the release of 450 jailed Palestinians.