( CNN ) -- For more than a year and a half , Lakhdar Brahimi was at the front lines of talks between the two sides of Syria 's civil war . As the U.N.-Arab League special envoy to Syria , he doggedly pursued a political solution to the crisis , with little to show for it . And now , Brahimi is resigning from his post , U.N. Secretary-General Ban Ki-moon said Tuesday . Ban accepted the envoy 's resignation `` with great regret , '' he said , standing by Brahimi 's side at U.N. Headquarters in New York . The resignation is effective May 31 . The announcement came as a truce between Syrian rebels and the government showed signs of faltering . Rebels in Aleppo on Tuesday fired upon pro-government enclaves in that town in retaliation for Syrian troops earlier firing upon the opposition in Homs , according to the rebels and an opposition group . Brahimi had been apologetic before for the failure of peace talks but remained a highly regarded mediator . `` The resignation of Lakhdar Brahimi is further proof of the failure of international efforts to end the horrific bloodshed in Syria , '' said Andy Baker , region program manager for Oxfam on the Syrian crisis . `` Perhaps it 's no coincidence that the peace envoy to Syria should step down when the prospect for a peaceful resolution seems as distant as ever . '' The truce began last Wednesday in the strategic and symbolic city of Homs , where opposition fighters and their families underwent an evacuation , the opposition Syrian Observatory for Human Rights in London said . Those 600 fighters and their families were evacuated to Addar al-Kabira . But government forces shelled Addar al-Kabira this week , according to the opposition . `` The regime 's shelling of the town of Addar al-Kabira north of Homs , to where rebels of Homs retreated in line with a deal organized by the U.N. , makes evident the falsity of the truces that the Assad regime seeks to promote , '' Nasser al-Hariri , a member of the Syrian Coalition , said in a statement Tuesday . He was referring to President Bashar al-Assad 's regime . On Monday , Syrian troops opened fire on al Waer , the last known opposition stronghold in Homs , killing at least one person , the Syrian Observatory for Human Rights said . `` We consider the shelling of Waer a violation of the agreement , and it shows the government is not abiding by the truce on the ground , '' a statement from the Ahl al Sham rebel command center said . `` We will respond to this violation by shelling Nubul and Zahraa , '' two government enclaves in Aleppo , the statement said . The rebel Ahl al Sham fighting coalition in Aleppo posted video of fighters firing rockets into the two Aleppo neighborhoods , known to be sympathetic to the government . Originally , the opposition had agreed to allow aid into these pro-government neighborhoods , both Shiite enclaves , in exchange for the safe passage of fighters out of Homs .
NEW : U.N.-Arab League secretary-general accepts the Homs envoy 's resignation . Syrian rebels and the government agreed to a truce Tuesday . On Monday , government troops fired upon an opposition neighborhood in Homs . On Monday , rebels shelled two pro-government neighborhoods in Syria , opposition says .