The Hamas government in the Gaza Strip voiced its rage on Thursday after Gaza militants fired two rockets into southern Israel Thursday, causing no injuries but further straining a shaky, week-old truce between Israel and the Gaza rulers.

The militant group Al-Aqsa Martyrs Brigade, affiliated with Hamas rivals Fatah, claimed responsibility for Thursday's Qassam strikes. In a statement carried by the official Palestinian news agency Ma'an, Hamas warned the militant factions against violating the terms of the cease fire with Israel, saying such violations harmed the Palestinian national interest. Hamas also threatened to take the necessary steps against the violators.

A Hamas spokesman said the "tahadiyeh" (calm) was a national Palestinian interest, and that anyone who tried to sabotage it was acting in an "anti-national" way. He called on Egypt, which brokered the cease fire agreement, to talk to those factions that are "motivated by anti-national politics."

The Hamas government spokesman added that harming the national interest for the purpose of settling personal scores "works to advance the interests of the Israeli occupation."

Earlier Thursday, Foreign Minister Tzipi Livni called for Israel to respond militarily to the Qassam fire by Palestinian militants in the Gaza Strip.

Speaking after two Qassam rockets struck the western Negev Thursday, Livni said "there was a similar breach of the truce several days ago."

"I don't care who fired. Every breach must be met with an immediate military response. I made my opinion clear to both the prime minister and the defense minister following the first infraction, and I will make clear to my foreign counterparts too," Livni went on to say at the start of her meeting with Norwegian Foreign Minister Jonas Gahr Stoere.

The Al-Aqsa Martyrs' Brigade, a militant group aligned to Palestinian President Mahmoud Abbas's Fatah faction, claimed responsibility for the attack and demanded that the cease-fire be extended into the West Bank.

"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 the group.

Israel said it would keep its border crossings with the Hamas-ruled Gaza Strip closed for a second day on Thursday, prompting the Islamist group to warn that the move could wreck the truce.

The crossings were opened on the day the truce went into effect last Thursday, but a decision was made to close them again Tuesday evening, following a violation of the cease-fire by Islamic Jihad militants firing Qassam rockets into Israel.

Two Israelis were lightly wounded during those attacks, as four Qassam struck the western Negev town of Sderot and the surrounding area.

Israeli military liaison official Peter Lerner said that the crossings 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.

Hamas spokesman Sami Abu Zuhri accused Israel of breaching the ceasefire that began a week ago.

"If this closure continues it will render the deal for calm meaningless," Abu Zuhri said.

"Securing the continuation of the Palestinian factions' commitment to the deal hinges on the Occupation's lifting of the siege and the opening of all the crossings in the first 10 days," he said, referring to Israel.

Defense officials expressed concern during Wednesday's meeting that a continuation of the border closure may result in a complete breakdown of the cease-fire agreement with Hamas.

However, following a reassessment of the security situation, it was decided to open the crossings over the weekend. The security assessment will also determine whether extensive quantities of goods and supplies will be allowed into the Gaza Strip.

Security officials said Wednesday that Israel would permit humanitarian cases to cross into Israel for medical assistance through the Erez crossing.

The Islamic Jihad militant group on Wednesday threatened to continue its rocket attacks on Israel, despite the truce.

Infrastructure Minister Benjamin Ben-Eliezer told Army Radio on Wednesday that Israel must respond to any continuation of Qassam fire with full force, despite the truce agreement.

"We will give them [Hamas] a chance, but if the Qassam fire on the Negev continues, Israel must respond in full force," Ben-Eliezer said.

Also on Thursday, Hamas forces shut down two smuggling tunnels running from southern Gaza to neighbouring Egypt. The area has long been a focus of concern for Israel in light of its assessments that Palestinians use tunnels to bring in weapons.

But Hamas said other kinds of contraband were being targeted and that the tunnel hunt, in which up to 40 of the secret passages could be closed, was not linked to the cease-fire.

"This is not a new campaign. It is an ongoing campaign to close down tunnels suspected of being used to smuggle drugs into our areas," said Interior Ministry spokesman Ehab al-Ghsain. "It has nothing to do with the calm agreement."

Israeli officials have said Israel's truce conditions included a demand that Cairo to do more to stop the flow of munitions from the Egyptian Sinai into Gaza.