Targeting leaders of anti-Israel Palestinian groups in Syria would be "an aggression that will be handled as an aggression," al-Asad said in an interview with Aljazeera aired on Saturday.
Israeli officials have vowed to kill officials of guerrilla groups responsible for "terror acts", one of the most prominent of whom is Khalid Mishaal, the political leader of the Islamist resistance group Hamas, who lives in Syria.
"Even if Israel did not make a threat, the threat is always there. No one trusts Israel ... the threat has been there since Israel was created," al-Asad said. "Israel expresses itself freely, not through the freedom of speech but rather through the freedom of killings," he said.
In the past six weeks, Israel has assassinated Hamas' spiritual leader and co-founder Shaikh Ahmad Yasin and its Gaza leader Abd Al-Aziz al-Rantisi.
The White House said on Friday it would impose sanctions on Syria soon for supporting what it described as terrorism and for failing to stop guerrillas from entering Iraq.
Syria describes the armed Palestinian and Lebanese groups that it supports as freedom fighters.
In his interview, al-Asad defended the Palestinian resistance against Israeli occupation forces and said it was legitimate "so long as it represents what the Palestinian people want. We do not have the right to evaluate it; only the Palestinians have that right".
He told Aljazeera there was no change in his country's longstanding position on the resumption of peace talks with the Jewish state.
"We have always said that we are ready to negotiate with Israel when the circumstances in Israel are suitable."
He also criticised US President George Bush for his recent pledge to Israeli Prime Minister Ariel Sharon that Israel could keep some West Bank land and that Palestinian refugees should not expect to reclaim their homes in what is now Israel.
Bush made the pledge when presented with Sharon's controversial plan to withdrew Jewish settlements from Gaza as part of a broader separation of Israelis from Palestinians.