Annan will officially step down on Aug. 31, The Guardian (Britain) reported.
"It is with deep regret that I have to announce the resignation of the U.N.-League of Arab States Joint Special Envoy for Syria, Mr. Kofi Annan," U.N. Secretary-General Ban Ki-moon said. " ... Tragically, the spiral of violence in Syria is continuing."
Meanwhile, opposition fighters in Aleppo have captured armored tanks and are for the first time using heavy weaponry against Syrian forces, human rights groups said.
Officials with the Syrian Observatory for Human Rights -- based in Britain -- said the opposition used the captured armor to shell a military airport near Aleppo.
The United Nations confirmed Wednesday rebel reports that the Syrian Army is using jet fighters to crush the opposition, The New York Times reported Thursday.
"Our observers confirmed fighter aircrafts firing rockets and cannons -- heavy machine-gun fire," said Sausan Ghosheh, a spokeswoman for the U.N. monitor mission.
"It's a rapid escalation," said Andrew J. Tabler, a senior fellow at the Washington Institute for Near East Policy. "Once you start using fixed-wing aircraft and you have a city under full revolt, it's clear that the Assad regime is not going to stop and is not breaking. We're entering a new phase of this conflict."
Meanwhile, the U.N. World Food Program on Wednesday supplied food assistance to 28,000 people in Aleppo, the Times reported. Residents said water, food and electricity are scarce in the region as government sources attempt to cut off supply lines.
The conflict in Syria began in March 2011 with peaceful protests against the regime of President Bashar Assad. Human rights monitors have documented atrocities committed by both sides throughout the uprising.