"Tanks are still shelling or storming towns and villages before going back to their bases," he added. "That does not mean they are withdrawing."
The assaults were taking place despite Assad's pledge to implement by April 10 a six-point peace plan brokered by UN-Arab League envoy Kofi Annan.
The Observatory has charged that the army was torching and looting rebel houses across the country in a campaign that could amount to crimes against humanity.
On Wednesday, 52 people were killed around the country, including 28 civilians, with most of the casualties in the city and province of Homs, said the Observatory.
The Syrian National Council, the main opposition bloc, accused the regime of carrying out "a policy of genocide against the Syrian people" and called for immediate pressure from the international community for a pullback of tanks.
Russia's Foreign Minister Sergei Lavrov, meanwhile, predicted that under-equipped rebel forces would never be able to defeat Syria's powerful military.
"It is clear as day that even if the Syrian opposition is armed to the teeth, it will not be able to defeat the government's army," the Interfax news agency quoted Lavrov as saying while on a visit to the ex-Soviet nation of Azerbaijan.
"Instead, there will be carnage that lasts many, many years -- mutual destruction."
Lavrov said two groups of Syrian opposition representatives will be visiting Moscow in the coming days and that Russia will try to convince them that it wants to help resolve the year-long crisis.
On Monday, Annan told the UN Security Council that Assad had agreed to "immediately" start pulling troops out of protest cities and complete a troop and heavy weapons withdrawal by April 10.
But the United States accused the Syrian leader on Tuesday of failing to honour his pledged troop withdrawal.
"The assertion to Kofi Annan was that Assad would start implementing his commitments immediately to withdraw from cities. I want to advise that we have seen no evidence today that he is implementing any of those commitments," US State Department spokeswoman Victoria Nuland said.
With international concern growing, a draft Security Council statement was drawn up asking Syria to respect the April 10 deadline, according to a copy seen by AFP.
The draft also urges the Syrian opposition to cease hostilities within 48 hours after Assad's regime makes good on its pledges.
It calls on all parties to respect a two-hour daily humanitarian pause, as called for in Annan's plan.
Negotiations on the text -- distributed by Britain, France and the United States -- began on Tuesday. France's UN envoy Gerard Araud said he hoped it would be adopted late Wednesday or on Thursday.
Russia, Assad's veto-wielding ally in the council, has rejected the idea of a deadline, with Lavrov saying "ultimatums and artificial deadlines rarely help matters."
Meanwhile, Annan is to address the UN General Assembly on Thursday about the crackdown.
General Assembly President Nassir Abdulaziz al-Nasser convened an informal meeting for 10:00 am (1400 GMT) for Annan to provide via video conference an update on the situation and the "progress of his mission."
Seeking to assuage humanitarian concerns, Syria's Foreign Minister Walid Muallem pledged Damascus would do its utmost to ensure the success of a Red Cross mission, at a meeting on Tuesday with visiting ICRC chief Jakob Kellenberger.
Kellenberger, who is pushing for a daily ceasefire, travelled on Wednesday to Daraa to assess the humanitarian needs there, the ICRC said.
Two lorries filled with food aid and hygiene kits, as well as 500 blankets, were unloaded at Red Cross depots in Daraa ready for distribution, ICRC spokesman in Damascus Saleh Dabbakeh told AFP.
The state news agency SANA, meanwhile, said "armed terrorist groups" had burnt down a Syrian Red Crescent depot in the Karabis district of Homs, in an incident condemned by the local relief organisation.