Palestinian President Mahmoud Abbas condemned the killings as a violation of an informal seven-month-old truce and vowed that the Palestinian Authority would do everything to prevent attacks.
Palestinian Interior Minister Nasser Yussef met Israeli Defence Minister Shaul Mofaz to resume coordination over the pullout, suspended after a deadly 12 July bombing, but his office said the meeting failed.
"The meeting ended in failure and had no results," the minister's spokesman Tawfiq Abu Khussa told AFP.
"We obtained no answers to our questions on the calendar or mechanism of withdrawal, nor on the future of the buildings and infrastructures of the settlements which are to be evacuated."
Also on Sunday, Palestinians in the Ain al-Hilwa refugee camp in south Lebanon blocked the entrances of the camp with stones and burning tyres in protest against the heavy security measures adopted by the Lebanese army around the camp.
The Lebanese army stepped up security after Defence Minister Elias al-Murr accused Palestinian groups of trying to assassinate him and hiding inside the camp.
On Saturday, US Secretary of State Condoleezza Rice, on a three-day visit to the region, had spoken of progress between the two sides in coordinating the withdrawal.
Sharon said that Israel would "not countenance" further violence and that he had conveyed his position to Rice at the weekend.
Secretary of State Condoleezza
Rice visited the region this week
"I made it clear to her the additional instructions that have been given to the security establishment on this issue are very clear and that the IDF (Israel Defence Forces) will respond sharply against terrorist activity," he told a cabinet meeting.
"Our responses will be with very sharp means whether terrorism is perpetrated during the evacuation or afterwards," he said.
The Israeli husband and wife, grandparents in their 50s, were killed when Palestinians fired on their car on Saturday as they set out for home after spending the sabbath with relatives in one of the settlements to be dismantled.
Another five Israelis, including one soldier, were wounded in the attack near the main border crossing between Gaza and Israel.
The ambush was claimed jointly by the armed wing of Islamic Jihad, the Al-Aqsa Martyrs Brigades, which is loosely affiliated with Abbas's ruling Fatah party, and the Popular Resistance Committee.
Afterwards a Palestinian from Islamic Jihad's armed Al-Quds Brigades and another from Al-Aqsa were shot dead by Israelis - one by guards protecting settlements and the other by soldiers during a search operation, sources said.
President Mahmoud Abbas said
the killings violated a truce
Abbas condemned the killing of the two Israelis as a violation of the informal truce and February's Israeli-Palestinian peace summit in the Egyptian resort of Sharm al-Shaikh.
"I condemn this operation and consider it a violation of the hudna (truce), of the Sharm al-Shaikh understanding and the Cairo agreement between all the Palestinian parties," he said in a statement.
"The Palestinian Authority will make every effort to stop these useless operations.
"This violation of the hudna gives the Israeli side reason to continue their attacks against our people and the assassinations of Palestinian leaders," he said.
Deputy Prime Minister Shimon Peres ruled out any delay to the pullout after the violence.
"We cannot let Palestinian extremists dictate the timetable," he said.
Meanwhile, Aljazeera's correspondent in Ram Allah said a fire broke out in a prison in southern Israel.
Detainees in Negev prison said a fire engulfed a number of camps in section A inside the prison.