Syria’s top diplomat to Britain defected Monday, according to the British Foreign Office, striking a blow against the beleaguered government of President Bashar al-Assad as tens of thousands fled heavy fighting in Aleppo, the most populous city in the country and its commercial capital.

 Khaled al-Ayoubi, the charge d’affaires at the Syrian Embassy in London, is one of several high-level officials to defect recently. Others include Manaf Tlas, a senior military figure and a confidant of Assad; Nawaf al-Fares, who had been ambassador to Iraq; the charge d’affaires in Cyprus; and the ambassador to the United Arab Emirates.

 “Mr. al-Ayoubi has told us that he is no longer willing to represent a regime that has committed such violent and oppressive acts against its own people, and is therefore unable to continue in his position,” the Foreign Office said in a statement. “We urge others around Bashar al-Assad to follow Mr. al-Ayoubi’s example.”

Ayoubi has been the top-ranking Syrian diplomat in London since Syria’s withdrawal of its ambassador and Britain’s expulsion of the acting head of the embassy.

 Although the defection was a symbolic setback, the Syrian government is trying to project an image of strength as it ramps up an offensive in Aleppo that has sent more than 200,000 people scrambling for safety in recent days, according to a statement from the United Nations’ chief of humanitarian assistance, Valerie Amos. Many more people could be trapped by fighting in the city, Amos said.

 The Syrian government sent a clear message Sunday that it doesn’t want any outside interference. The army attacked the convoy of the new head of the U.N. observer team, Lt. Gen. Babacar Gaye, outside Damascus. “Yesterday, the convoy of General Gaye was attacked by army tanks,” U.N. Secretary General Ban Ki-moon told reporters in New York. “Fortunately, there were no injuries.”

Gaye visited Rastan and Homs on Sunday, where he witnessed the fighting and infrastructure damage first-hand and met with members of the rebel Free Syrian Army to discuss the possibility of political dialogue.

Col. Qassem Saad Eddine, a Free Syrian Army Joint Command spokesman who met with Gaye, posted a video statement online Sunday describing an outline for a political transition after Assad. The plan calls for power-sharing between leaders of the Free Syrian Army and the civilian opposition. Political and military councils would be created to oversee the drafting of a new constitution and elections.

 The Syrian military continued its assault in Aleppo on Monday, with planes bombing residential areas and helicopters firing rockets at rebel positions, according to the Local Coordination Committees, an activist network. One of the neighborhoods hit hardest was Salahuddin, where fierce street fighting broke out between rebels and government soldiers, according to opposition groups. A video circulated by the activist network purported to show the bodies of young men lying in pools of blood in a building there.

Both sides claimed to be making progress in their battle to control the city, but those assertions were impossible to confirm. The rebels appeared to take control of a government military base outside Aleppo after an hours-long overnight battle, CNN reported, giving them access to some heavier weapons.

For those left in Aleppo, life is becoming increasingly grim. Residents fleeing heavy fighting in some neighborhoods have taken shelter on the campus of Aleppo University; some are sleeping in parks or in their cars. Fuel and bread prices have spiked and electricity and water service goes out frequently, residents said.

 “Who cuts the water in Ramadan?” asked a 20-year-old man from Aleppo in a chat on MSN Messenger, referring to the Muslim holy month. “People are fasting! This is sinful.” Another young man in Aleppo said he went to get his passport renewed Sunday and was told that the head of the office had been kidnapped. “The situation escalated here before I managed to leave the country,” the 24-year-old man said in an MSN Messenger chat.

 Meanwhile, opposition groups reported that the Syrian army shelled a handful of neighborhoods in Damascus on Monday, a day after Foreign Minister Walid al-Moualem said the rebels had been defeated in the capital.

 At least 90 people were killed in fighting across the country Monday, including 26 people in Damascus and 25 in Aleppo, according to the Local Coordination Committees. Elsewhere, the Assad government closed its embassy in Australia, two months after that country’s foreign minister announced a departure deadline for two of Syria’s top diplomats in the wake of the massacre of dozens of civilians in the Houla area of Syria, according to ABC, Australia’s national broadcaster.

Ahmed Ramadan and Suzan Haidamous contributed to this report.