Hamas rejects the truce between Israel and Palestinians
The death of Ahmed Marshoud follows the shooting on Sunday of a prominent Hamas member by Israeli troops.
Marshoud, 32, died outside the offices of Palestinian leader Yasser Arafat's Fatah movement in Nablus, Palestinian security officials said.
It is unclear what caused the explosion. The French news agency AFP reported it was a bomb in or near a car, while Reuters quoted witnesses saying a rocket fired from an Israeli helicopter hit the car.
Two other people were injured in the blast, officials said.
Adnan Asfur, a local Hamas leader, accused Israel of carrying out its second assassination of a Hamas member in two days.
"This is a crime which confirms that the Israeli Government's policy of a ceasefire is to continue assassinating Palestinian freedom fighters," he said.
The Israeli army declined to comment.
The death brings the number of people killed in the year-long Palestinian uprising, or intifada, to 875. Of those, 676 were Palestinians and 176 Israeli.
Palestinian sources said Marshoud had spent some years in an Israeli jail for security offences but details were not immediately available.
On Sunday, Abdul Rahman Hamad was shot dead by an elite Israeli army unit as he stood on the flat roof of his house in the West Bank town of Qalqilya.
Hamas targeted before
Israel accused him of organising the suicide bombing of a Tel Aviv disco in June, in which 22 people died.
The shooting was the first time Israel had reverted to its policy of killing suspected militants since a truce was agreed on 26 September.
Hamas rejects the ceasefire agreed by Yasser Arafat and Israeli Foreign Minister Shimon Peres, saying it will not compromise with a Jewish state it does not recognise.
As part of efforts to keep the fragile truce, Israeli troops withdrew from positions in Palestinian-controlled parts of the West Bank city of Hebron on Monday and the army said it would ease a blockade on Palestinians.
America has stepped up pressure on both sides to reach a deal as it seeks Islamic support for its anti-terror coalition after the 11 September suicide hijackings in Washington and New York.