Syrian forces and oppsition fighters have clashed violently in and around Aleppo as the battle for control of the country's largest city raged into a fourth day, with both sides claiming advances.

The Syrian army said on Monday it had overrun part of the city's opposition-held Salaheddin district, which sits in the southwest in the face of the government advance, but that claim was denied by a Free Syrian Army commander.

Colonel Abdel Jabbar al-Oqaidi, head of the opposition military council in Aleppo, insisted government troops had "not progressed one meter".

"We launched a new assault from Salaheddin during the night, and we destroyed four tanks," the opposition commander said by phone.

Hospitals and makeshift clinics in opposition-held eastern neighborhoods were filling up with casualties from a week of fighting in the city, a commercial hub drawn into the 16-month-long revolt against President Bashar al-Assad.

"Some days we get around 30, 40 people, not including the bodies," said a young medic in one clinic. "A few days ago we got 30 injured and maybe 20 corpses, but half of those bodies were ripped to pieces. We can't figure out who they are."

Omar Khashram, an Al Jazeera reporter, was injured by shrapnel when a mortar round fell near the car he was riding in. Other Al Jazeera journalists in the same car were unhurt. Khashram was hit in areas unprotected by his flak jacket and was evacuated safely to Turkey.

The Syrian Observatory for Human Rights, an activist group in London, said 40 people, 30 of them civilians, were killed in Syria on Monday, and two opposition fighters died in Salaheddin.

Since the government's assault began on Saturday, the opposition fighters "already repelled three offensives" against Salaheddin, Oqaidi said, adding that they control "between 35 to 40 per cent of Aleppo".

Rami Abdel Rahman, the head of the Observatory, said government forces were on Monday "just reaching the outskirts of the [Salaheddin] neighborhood".

Colonel Oqaidi said that "several quarters of Aleppo are being bombed with MiG [fighter jets] and helicopters".

Activists on the ground reported that a string of opposition-held districts in Aleppo were shelled or hit by small-arms fire during the night. They said some were strafed by helicopter gunships.

UN convoy 'attacked'

UN Secretary-General Ban Ki-moon on Monday singled out Syria's government for censure, telling it to halt its violent crackdown on the dissidents and opposition fighting to overthrow President Bashar Assad's regime, a day after Syrian tanks attacked a convoy carrying the head of the UN observer mission.

Ban noted that his peacekeeping chief, Herve Ladsous, recently went to Syria where he "called on the government to end its use of heavy weapons and demonstrate the commitment to ending the armed violence. Government officials have repeatedly said they would honor these commitments. We are still waiting for them to act."

Ban said that a dozen armored vehicles used by the military observers have been destroyed by blasts and shelling, and a convoy carrying the general heading the force was targeted on Sunday.

"Yesterday, the convoy of Lieutenant-General Babacar Gaye was attacked by armed tanks. Fortunately there were no injuries," Ban said.

The UN observer mission's latest 30-day mandate is due to expire in August, and the United States, Britain and France do not want to see it extended under these circumstances.

Syria's most senior diplomat in the UK, meanwhile, stood down in protest at the "violent and oppressive" actions of al-Assad's regime, the British foreign office confirmed on Monday.

In a blow to Damascus, charge d'affaires Khaled al-Ayoubi informed the British authorities that he was "no longer willing" to represent the government amid the brutal repression of Syrian people.

The British Government said his decision represented the "revulsion and despair" of Syrians and urged others around the world to follow his example and abandon Assad.

International concern

Outside of Aleppo, opposition forces seized a strategic checkpoint on the road from Turkey after a 10-hour battle, securing them free movement between the northern city and the border, an FSA commander said.

"The Anadan checkpoint ... was taken this morning after 10 hours of fighting," said General Ferzat Abdel Nasser, an opposition officer who deserted the Syrian army a month ago.

Six soldiers were killed and 25 were taken as prisoners, General Ferzat said, adding that four of his own men died in the fighting.

UN humanitarian chief Valerie Amos said in a statement that an estimated 200,000 people had fled from Aleppo in two days and an unknown number were still trapped in the city.

Amos said in New York on Sunday that she was "extremely concerned by the impact of shelling and use of tanks and other heavy weapons" on civilians in Aleppo, Damascus and other locations.

She said that many people in Aleppo had sought shelter in schools and other public buildings. "They urgently need food, mattresses and blankets, hygiene supplies and drinking water."

PHOTO CAPTION

Map of Aleppo locating shelling and fighting between army and opposition fighters in Syria's commercial hub.

Al-Jazeera