DAMASCUS (AFP) - The Syrian army sent reinforcements to Aleppo on Wednesday to join the intensifying battle for the country's second city, as the United Nations pulled out half of its troubled observer mission.

UN peacekeeping chief Herve Ladsous said he had stressed to Syrian officials in morning talks that without a significant reduction in violence, the remaining 150 observers would leave on the expiry of the "final" 30-day extension of the mission's mandate agreed by the Security Council on July 20.

Russia meanwhile ramped up its criticism of Western policy, accusing Washington of justifying terror by failing to condemn a July 18 bombing that killed four top Syrian security officials, and accusing the European Union of imposing a unilateral blockade on its Soviet-era ally.

Helicopter gunships strafed several neighbourhoods of Aleppo, Syria's commercial capital, causing deaths and injuries, the Syrian Observatory for Human Rights said.

Clashes raged in the central Al-Jamaliya neighbourhood, close to the local headquarters of the ruling Baath party. In Kalasseh, in the south of the city, rebels set fire to a police station, the Observatory said.

Fighter jets overflew the city, breaking the sound barrier but not carrying out bombing raids, Observatory director Rami Abdel Rahman told AFP.

A "large number" of troops have been moved from the northwestern province of Idlib to join the fighting in Aleppo, a rebel spokesman told AFP via Skype.

Free Syrian Army Colonel Abdel Jabbar al-Oqaidi said he believed the reinforcements were being sent because of the intensity of clashes in Aleppo, where several districts were "liberated" on Monday.

"There are clashes right now in Aleppo, so fierce that many of their troops are running away, while dozens of others are defecting on the spot," Oqaidi said. "Their morale is very low."

The Britain-based Observatory also reported clashes in the district of Al-Hajar al-Aswad in Damascus, one of the last remaining rebel bastions in the capital after 10 days of fighting.

Regime forces used helicopter gunships and heavy machinegun fire to pound the embattled southern neighbourhood, as they tried to "reclaim" it, the Observatory said.

Nationwide at least 32 people were killed in violence, the majority of them civilians, after 158 people died on Tuesday, the watchdog added.

In Hama province in central Syria, a couple and their two children were killed as they tried to flee shelling in their village.

A video distributed by the Observatory showed grisly footage of the bodies of the family members, shrapnel tearing open the faces of the children and ripping open the head of their father. As the violence raged, the UN Supervision Mission in Syria, deployed to supervise an April truce that never took hold, sent home half of its 300 unarmed military observers.

"UNSMIS (is) in a reduced format," said Ladsous, the UN peacekeeping chief, who flew into Damascus on Tuesday.

"About half the military observers have been for the time being sent back to their countries, so the mission operates on a reduced basis, reduced in numbers, reduced in team size in the provinces and does what it can," he said.

"But of course taking into account the security situation, which of course in many places is extremely delicate."

Ladsous said he had stressed to Syrian officials that any extension of the observer mission beyond the 30 days set by the Security Council would require "very specific and sustainable progress on the level of violence, which should subside substantially, and on the use of heavy weapons."

Western governments have expressed scepticism about the chances that violence will subside sufficiently within the period of the mission's 30-day mandate but Russia, which has been increasingly critical of Western policy, has argued the observers should stay.

Russian Foreign Minister Sergei Lavrov on Wednesday lashed out at the United States for backing the armed opposition, saying Washington's failure to condemn the July 18 bombing meant it was justifying terror.

"This is quite an awful position, I cannot even find the words to make clear how we feel," Lavrov told reporters. "This is directly justifying terrorism. How can this be understood?"

And a Russian foreign ministry statement said that a new round of EU sanctions agreed this week, which allows for the inspection of vessels and planes suspected of carrying arms, amounted to an air and sea "blockade".