He told a group in Damascus on Monday that "no one" was capable of bringing about the necessary circumstances for a planned truce to take effect later this week.
"We hear about them requesting a ceasefire within a week. OK, then who is capable of bringing together all these conditions within a week? No one. Who will speak to the terrorists if a terrorist organization refused to adhere to the ceasefire, who will make them accountable? Who, as they say, will bomb them?"
In comments to Syria's central Bar Association, reported in Syria's state news agency SANA, Assad said many questions remain about which groups in the conflict could be classified as terrorists.
"As a state, anyone who bears weapons against the state and against the Syrian people is terrorist, and this is indisputable," he said.
U.S. questions Russia's willingness for ceasefire
Meanwhile, the U.N. said convoys are expected to start delivering aid in the coming days to besieged areas inside Syria after receiving authorization from the Assad government.
Vanessa Huguenin, a spokeswoman for the U.N. Office for the Coordination of Humanitarian Affairs, told CNN the convoys would be going into seven areas in Syria.
The Syrian Observatory for Human rights reported that humanitarian aid is expected to enter the cities of Zabadani, Madaya and Muadamiyat al-Sham in the Damascus suburbs Wednesday, as well as the Shiite towns of Kefraya and Foua in the suburbs of Idlib.
Critics have expressed doubts about the prospects of the proposed truce as the Syrian army, backed by Russian air power, pursues a major offensive in northern Syria.
U.S. national security adviser Susan Rice told reporters Monday that the escalating military campaign called into question "Russia's willingness or ability to implement the agreements achieved in Munich."
"The intensified bombings, the displacement, the fact that civilian entities have been hit by the regime and its backers, is of grave concern," she said.
Four hospitals and a school were struck Monday in the northern Syrian provinces of Aleppo and Idlib, the Office of the U.N. High Commissioner for Human Rights said Tuesday, killing at least 46 people and injuring scores of others.
A United Nations spokesman called the strikes a "blatant violation of international laws," France and Turkey labeled them war crimes, and Britain said they could amount to war crimes and must be investigated.
Western powers blamed Syria and its military ally Russia for the strikes on the hospitals; Russia denied the accusation Tuesday, and a Syrian diplomat said warplanes from the U.S.-led coalition were behind an attack in Idlib.
In one attack in Azaz in northern Aleppo province, 13 people were killed and dozens injured Monday when a hospital for women and children was struck, according to the U.N. human rights office.
Seven people were killed and 23 injured in another strike on a general hospital in Azaz, and 14 people were killed in a strike on a school in the town Monday, the U.N. human rights office reported.
About 100 kilometers (62 miles) away in Maarat al-Numan in Idlib province, nine people were killed and 30 injured when a hospital supported by Doctors Without Borders was struck, the U.N. human rights office said.
Missiles hit that facility four times within minutes, the humanitarian group said, leaving it in ruins.
Another hospital in the town was also struck Monday, leaving three dead and six injured, the U.N. human rights office said.
Airstrikes on February 5 killed three people and wounded at least six at a Doctors Without Borders-supported hospital in Daraa governorate in southern Syria, the aid group said.
Officials around the world traded accusations over who was behind the attacks.
The United States, the UK and France blamed Syria and its Russian supporters for the attacks, which French Foreign Minister Jean-Marc Ayrault said constituted war crimes.
British Foreign Secretary Philip Hammond said that Russia needed "to explain itself, and show through its actions that it is committed to ending the conflict, rather than fueling it."
Turkish Prime Minister Ahmet Davutoglu blamed Russia
for the strikes in Azaz, which is near the Turkish border, claiming Moscow had targeted the complex with ballistic missiles fired from the Caspian Sea.
But Kremlin spokesman Dmitry Peskov denied the accusations, according to Russian state news agency RIA Novosti.
And Syria's ambassador to Russia, Riad Haddad, said that the United States and coalition forces were responsible for the Idlib attack and that Moscow had intelligence proving U.S.-led coalition warplanes raided the hospital.
The United States has said it carried out no military operations in the attacked area.
Germany: Support for no-fly zone
Northern Syria has been the scene of intense fighting recently, displacing tens of thousands of people as Syrian regime forces, backed by Russian jets, pursue a major offensive on the key city of Aleppo, and Turkey shells Kurdish People's Protection Units, or YPG, near Azaz.
Turkey considers the YPG to be terrorists, while the United States backs the group in the fight against ISIS
Ankara has long advocated the creation of a no-fly zone in northern Syria, despite its bombing of Kurdish positions in the region.
A Turkish official told reporters in Istanbul on Tuesday that Turkey would not conduct a unilateral military operation in Syria, but would participate in a joint ground operation with coalition partners.
"We want a ground operation with our international allies," the official said.
Turkish Defense Minister Ismet Yilmaz said Sunday that his country has no intention of sending ground troops into Syria
, amid growing international concern about Ankara's shelling of Kurdish forces.
The official added that Saudi Arabia would deploy warplanes to Incirlik Air Base in southern Turkey by the end of the month, in a sign of a reinvigorated Saudi commitment to the U.S.-led coalition air campaign against ISIS.
German Chancellor Angela Merkel has also spoken of her support for the creation of a no-fly zone in northern Syria, telling German newspaper the Stuttgarter Zeitung
that it could create a haven for fleeing families, and offer at least a partial solution to Europe's refugee crisis.
"(I)t would be helpful if there were areas there, where no party to the war flew bombing missions -- in other words a kind of no-fly zone," she was quoted as saying as she responded to a reporter's question about her position on exclusion zones.
"We can't negotiate with the terrorists from ISIS. But if it were doable to come to such an agreement with the anti-Assad coalition and the Assad supporters, it would be helpful."