Syria’s army on Wednesday reportedly suffered its deadliest day in a ceasefire when rebel fighters killed 20 troops, in the latest violation of the three-week truce the UN says both sides are flouting.

The rebels killed 15 soldiers — including two colonels — in a dawn ambush in the northern province of Aleppo, where two rebel fighters also died, said the Syrian Observatory for Human Rights.

The ambush occurred near Al-Rai village, after President Bashar al-Assad’s forces had “scaled up military operations” there in the days since the truce took effect on April 12, said the watchdog.

Clashes near Damascus killed six troops, while the army shelled and torched activists’ homes in eastern Deir Ezzor province and regime gunfire killed a civilian in southern Daraa, cradle of the 14-month uprising.

The latest bloodshed comes a day after the United Nations accused both the regime and its opponents of violating the ceasefire that is part of a peace plan brokered by UN-Arab League envoy Kofi Annan.

The plan calls for a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate and the release of detainees.

According to the UN, more than 9,000 people have been killed in Syria since an anti-regime uprising broke out in March last year, while the Observatory puts the figure at more than 11,100.

UN peacekeeping chief Herve Ladsous said Syrian troops have kept heavy weapons in cities, and that both the government and rebels have violated the truce.

He also said UN members had so far only offered only 150 military observers for the 300-strong planned force and that Syria had refused visas for three proposed monitors.

But Syrian foreign ministry spokesman Jihad Makdisi denied visa requests had been turned down and said the two sides had agreed on the nationalities that could operate in the country.

“We agreed with the UN negotiating team that nationalities of observers to be mutually agreed upon ... So there is no refusal per se ... There are far more than 110 nationalities that can easily work in Syria,” he told AFP.

Ladsous said 24 monitors were currently in place.

“Regarding the heavy weapons, yes, our military observers do see a number of APCs (armoured personnel carriers), for instance; they see a number of Howitzers and other military equipment in most places where they are,” he said.

Syria had told the monitors the armoured carriers had been disarmed, but this had not been verified, added Ladsous.

“The important fact is that violations do come from both sides,” he said while refusing to say whether one side had committed more breaches.

“All the parties need to take further steps to ensure a cessation of violence in all its forms,” he said.

Human Rights Watch accused the regime of committing atrocities in the eastern province of Idlib shortly before the truce took effect.

“Syrian tanks and helicopters attacked one town in Idlib after another,” Anna Neistat, associate director for programmes and emergencies at HRW, said in a statement.

“It was as if the Syrian government forces used every minute before the ceasefire to cause harm,” she added.

The New York-based watchdog accused regime forces of summary executions, arbitrary detentions and burning and destruction of civilian property.

In some of the incidents recorded by the global rights watchdog, children were executed by regime forces.

“The security forces also arbitrarily detained dozens of people, holding them without any legal basis,” HRW said.

According to one eyewitness account published by HRW, the mother of Mohammed Saleh Shamrukh, an anti-regime protester from Saraqeb, in Idlib, had to watch regime forces take him away.

“I didn’t say goodbye so as to not make him sad. He didn’t say anything either. When they left, the soldiers said I should forget him,” she said.

Shamrukh was executed on March 25.

Another woman recounted how regime forces entered her home in the town of Taftanaz searching for her husband.

“They put a Kalashnikov to my head and threatened to kill us all if my husband did not come home,” she said.

“Then an officer told a soldier to get petrol and told the children that he would burn them like he would burn their father because he is a terrorist.”

She said she was finally allowed to leave the house before it was burned down on April 4 along with the houses of her five brothers-in-law.

HRW said that during the April 3-4 attack on Taftanaz, northeast of Idlib city, 19 members of the Ghazal family, including two under the age of 18, were executed by regime forces. Nine males were shot in the head or back.

Meanwhile, the International Monetary Fund said Syria’s economy was likely to contract significantly in 2012 due the violence and international sanctions against the Assad regime.