Khaled al-Ayoubi, the charge d'affaires at the Syrian mission, told the British Foreign Office he was unable to "represent a regime that has committed such violent and oppressive acts against its own people." He was in discussion with British officials, at a safe location and with his immediate family, the London newspaper The Daily Telegraph reported.
The departure of al-Ayoubi, an ethnic Kurd who joined the Syrian diplomatic service in 2001, is the latest in a series of military and diplomatic defections as the rebellion against Bashar Assad's government heads into its 18th month, the newspaper said.
The British Foreign Office called his departure "another blow to the Assad regime," adding "It illustrates revulsion and despair the regime's actions are provoking among Syrians from all walks of life, inside the country and abroad."
In Syria, rebels took over a government military base outside of Aleppo Monday that had about 200 Syrian troops, activists said.
Activists also reported at least 12 people were killed across Syria Monday, including eight in Aleppo, the country's largest city that has seen more than a week of fighting, CNN reported.
Besides Aleppo, activists reported shelling in the Damascus suburbs of Deraa, Homs and Idlib, The Guardian reported.
About 200,000 people in and around Aleppo have fled in the past two days, Valerie Amos, the U.N. undersecretary-general for humanitarian affairs, said Sunday.
French Foreign Minister Laurent Fabius said Monday his country will step up its efforts to stop the bloodshed, CNN reported.
"As France is taking over the presidency of the U.N. Security Council on Aug. 1, we are going to ask -- before the end of the week -- for a meeting of the Security Council, probably at a ministerial level ... to try and stop the massacres and prepare for the political transition," Fabius said on French Radio.
Meanwhile, Syrian Foreign Minister Walid Moallem, during a trip to Iran, Sunday predicted the rebels' battle for Aleppo would fail, CNN said.
"Since last week, [opposition fighters] planned for whatever they called the 'great Damascus battle,' but they have failed after one week," Moallem said, referring to a failed rebel offensive earlier this month. "That's why they moved to Aleppo, and I can assure you that they will fail."
U.S. Defense Secretary Leon Panetta predicted Sunday the Syrian regime's violent crackdown in Aleppo will be "a nail in Assad's coffin" by turning even more people against Assad and his government.
Arab League chief Nabil Elaraby said Sunday the league believes war crimes are being committed in the city.
The leader of the Syrian National Council, a prominent Syrian opposition group, urged world leaders to help arm the rebels.
"Our friends and allies will bear responsibility for the terrifying massacres that will happen in Aleppo if they don't move soon," Abdulbaset Sieda said. "The rebels now are fighting with primitive types of weapons against the killing machine. We need weapons that will allow us to stop tanks and planes."
Iranian Foreign Minister Ali Akbar Salehi accused Israel of orchestrating "a conspiracy against Syria" and issued a warning to global leaders, CNN said.
"It is completely ridiculous and delusive to believe that there is a possibility of creating a vacuum in the leadership in Syria," Salehi said. "We call upon the people of the region to be fully aware and not to move in the wrong direction because there will be severe consequences that will go beyond the borders of the region to the outside world."
Moallem also criticized Israel, along with Qatar, Saudi Arabia and Turkey, for the escalation of violence.
Salehi, whose regime is Syria's lone regional ally, said Qatar, Saudi Arabia and Turkey were "wrong, naive and deluded" if they thought Assad's removal would bring about a Syrian government friendly to their interests, adding, "If they continue moving in the wrong direction, then let them rest assured that the consequences of this will affect them too."
Last week, the United Nations said nearly 17,000 people have died since the crisis began March 2011.