"In principle, we expect that there will be an Israeli aggression at any time. We all know that Israel is militarily powerful and is backed directly by the United States," al-Assad said in an interview with Kuwait's Al-Anbaa daily published on Saturday.
"We can't debate whether to be prepared or unprepared. We must remain always prepared."
The Syrian president said Israel had abandoned the Middle East peace process since [former prime minister] Ariel Sharon came to power in 2001.
"This means that there will be no peace in the foreseeable future. If there is no peace, naturally you should expect that war may come. The no-war, no-peace situation means there will either be peace or war."
Syria placed itself on alert for an Israeli attack during Israel's 34-day offensive against Hezbollah in Lebanon which ended in mid-August. Hezbollah is supported by Syria and Iran.
Separately, Mohsen Bilal, Syria's information minister, said his country had taken into account the possibility of an Israeli military attack. "Syria has considered the possibility of Israeli military action," Bilal told Aljazeera on Sunday.
He said Syria "is ready for all options and for Israel's hostile plans following the failure of its aggression in Lebanon".
Bilal also described the government of Ehud Olmert, the Israeli prime minister, as "in crisis after its defeat in Lebanon and the victory of the Lebanese resistance".
A political adviser to Amir Peretz, Israel's defence minister, said the Syrian position could not be ignored.
"Assad's declarations must be taken seriously," Reserve General Amos Gilad said.
"It is necessary to carefully study such menacing declarations. In the long term the declarations of Assad are of great importance, but for the time being they change nothing. It is not a concrete threat."