JERUSALEM – Clashes and mortar fire in the West Bank and Gaza Strip killed six people yesterday, one of the deadliest days of violence since Israel and the Palestinian Arabs declared a cease-fire four months ago.

While the two sides said they would still observe the truce, the fighting raised already heightened tensions and threatened nascent efforts to coordinate Israel’s upcoming withdrawal from the Gaza Strip with the Palestinian Arabs.

“The situation is deteriorating. The whole cease-fire may collapse,” a Palestinian Arab negotiator Saeb Erekat, said, calling for international intervention.

The day’s violence included a West Bank shootout in which two Islamic Jihad members were killed, a mortar barrage that killed three non-Israeli laborers in a Jewish settlement in Gaza, and the killing of a man who infiltrated Gaza from Egypt. Palestinian Arab terrorists also fired an anti-tank missile and seven homemade rockets at Israeli targets, causing damage but no injuries.

Israel issued a stern warning for the Palestinian Arabs to rein in the fighters; the terror groups Hamas and Islamic Jihad said Israel was responsible for the fighting.

Israeli Foreign Minister Silvan Shalom accused Hamas of trying to derail improving ties with the Palestinian Arabs. “Hamas is trying very hard to undermine our efforts to move toward peace with the Palestinian Authority. Hamas is trying to undermine Abu Mazen’s regime as well,” he said, using the nickname of the Palestinian leader Mahmoud Abbas.

Mr. Shalom spoke alongside the British foreign secretary, Jack Straw, who accused Hamas and other groups of using “wanton, random terror” to weaken Mr. Abbas.

Mr. Straw said his government would have “no dealings” with Hamas’s leadership until the group renounces violence, despite admitting earlier in the day that British diplomats recently met with Hamas-affiliated politicians.

Touring Sderot, an Israeli town just outside Gaza where a rocket hit a house yesterday, the Israeli military commander, Lieutenant General Dan Halutz, accused the Palestinian Authority of not using its ability to stop the terrorists.

Palestinian Arab officials “think the right things and say the right things but don’t do the right thing,” he said, allowing terrorists to use attacks against Israel as part of their own struggle with Palestinian Arab officials.

Hamas said the Sderot attack was retaliation for Monday’s visit by Jews to a disputed holy site in Jerusalem, setting off a clash. “We are committed to the truce, but at the same time we have to respond to any violation,” a Hamas spokesman, Mushir al-Masri, said.

Prime Minister Sharon and Mr. Abbas declared the cease-fire on February 8 in a bid to end more than four years of bloodshed. Since then, there has been a sharp drop in violence, though there have been occasional flare-ups.

Yesterday, Israeli soldiers killed a top Islamic Jihad terrorist in the West Bank town of Jenin. The head of the group’s local military wing, 25-year-old Morwah Kamil, was shot in a gun battle that erupted after Israeli troops entered the nearby town of Qabatiya on an arrest raid, the army and witnesses said.