U.N. peacekeeping chief Herve Ladsous says the conflict in Syria is now a full-scale civil war, AFP reports.
The comments come after reports that the Syrian government has lost "large chunks of territory" in some cities.
The U.N. also reported members of its observer team in Syria were attacked by angry crowds as they tried to reach an embattled mountain village near President Bashar Assad's hometown. The vehicles also were later shot at.
Annan says the attacks against the observers Tuesday were deliberate, Reuters reports.
None of the observers were injured.
The global body says the observers' vehicles were surrounded Tuesday before they could reach rebel-held Haffa -- a village about 20 miles from Assad's hometown of Kardaha. The U.N. says the crowds hurled stones and metal rods at the vehicles, which then turned back.
It says shots were fired at the three vehicles as they were heading toward Idlib.
The U.N. says the crowds appeared to be residents of the area but the source of the gunfire was unclear.
On Tuesday, at least 10 people died as Syrian forces hit the eastern city of Deir el-Zour with mortar shells after an anti-government protest, and clashes broke out elsewhere in the country, activists said.
The International Committee of Red Cross said that its impossible to respond to all humanitarian needs at once due to simultaneous fighting that has spread to several parts of the country, Reuters reports.
The ICRC is trying to locate hundreds of citizens believed to have fled throughout Homs in search of safe areas, a spokesman told Reuters.
Syria has intensified its onslaught against the opposition in recent days, ignoring an Annan-brokered ceasefire plan, mounting international condemnation and increasing economic pressure aimed at the government of President Bashar Assad.
Annan, the former U.N. Secretary-General, has been working with little success to end the Syrian conflict since he was appointed in February
The Associated Press contributed to this report.