Four Qassam rockets fired from Gaza struck Sderot and the surrounding area Tuesday afternoon, further imperiling a brittle truce declared six days ago.

One of the rockets struck a vacant house in Sderot and two others hit the city. A fourth rocket exploded in an open area in the Sha'ar Hanegev Regional Council.

Two people was lightly wounded in the barrage, and two others were treated for shock.

Islamic Jihad claimed responsibility for the Qassam salvo. Gaza's Hamas rulers issued a statement saying that they were still committed to the cease-fire with Israel, despite the small militant group's obvious violation of its terms.

Sami Abu Zuhri, a Hamas official, responded to the attacks by urging "all Palestinian factions to abide by the calm agreement," adding: "Hamas is keen to maintain the deal."

Late on Monday night, Palestinians fired a mortar shell at the Negev, in the first such strike since a cease-fire went into effect in the Gaza Strip last week.

IDF troops kill two in West Bank raid

Meanwhile, Israel Defense Forces troops operating in the West Bank city of Nablus killed two Palestinian militants early Tuesday morning.

One of the dead was a senior Islamic Jihad militant, and the other belonged to Hamas.

The raid took place near An-Najah University, in Nablus. Neighbors said they found the men riddled with bullets and shrapnel after the raid.

The militants were named as 24-year-old Iad Hanfar, of Hamas, and the second as Taker Abu Rali, a senior Islamic Jihad militant.

According to the IDF, Abu Rali was planning an attack on Israel. Troops discovered ammunition, explosives and rifles in his apartment.

In a statement released from the West Bank following the raid, Islamic Jihad said that "the reprisal for this noble blood will be in the depths of the Zionist entity, God willing."

An Egyptian-brokered cease-fire agreement was struck between Israel and Hamas last week, but the deal extends to the Gaza Strip only, leaving the IDF free to operate in the West Bank.

While Islamic Jihad said it would abide by the cease-fire, the group has also said it would reserve the right "to respond to the Zionist violations" of the truce.

"Any aggression, in the Gaza Strip or West Bank, requires a response by the resistance," Abu Ahmed told Haaretz last week. "But the mechanism of response, its place and time, will be decided according to the nature of the aggression."

Nablus's governor Jamal Muheisen called the IDF raid in the city an "unjustified crime" but said he did not believe it would threaten the Gaza truce.

In Gaza, a Hamas spokesman group accused Israel of trying to disrupt the atmosphere of calm that started five days ago.

"The resistance factions in the West Bank have the full right to respond to this crime," spokesman Fawzi Barhoum said.

Security forces loyal to Palestinian President Mahmoud Abbas were deployed in Nablus late last year as part of a Western-backed law and order campaign coordinated with Israel.

But local Palestinian commanders say frequent IDF raids into the city have undercut that effort.

Also on Tuesday, Al-Sharq Al-Awsat, a London-based Arabic-language daily, quoted Prime Minister Ehud Olmert as saying that if the smuggling of weapons into Gaza does not cease, Israel will consider the cease-fire agreement violated, and will be forced to respond militarily. Hamas has said it will not stop smuggling weapons.