Syrian opposition fighters in the besieged city of Aleppo have 'tanks' and 'heavy weapons', the UN spokesman said on Wednesday.
International pressure must be put on the Syrian opposition, as well as President Bashar al-Assad, to halt the fighting, UN spokesman Martin Nesirky said.
UN observers "have confirmed information that the opposition is in possession of heavy weapons including tanks in Aleppo," Nesirky told reporters at UN headquarters.
He said they were mainly weapons that had previously belonged to the Syrian armed forces. Unarmed observers from the UN Supervision Mission in Syria (UNSMIS) went to the northern city on Tuesday.
Aleppo has been under siege from Assad's forces for more than a week. UNSMIS has also reported seeing a fighter jet attacking the city.
"The UN has reminded both parties of their obligation under international humanitarian law to protect civilians," Nesirky said.
More than 200,000 people have fled Aleppo, according to the UN, and Nesirky said many of the inhabitants remaining have taken shelter in schools and other public buildings.
Nesirky stressed that UN leader Ban Ki-moon wants united international pressure on both sides in the civil war.
He said pressure should be brought to bear on "not just the Syrian government forces - who of course bear the lion's share of the responsibility for what is happening - but also on the opposition forces, to ensure that they do heed the calls, that they do stop the fighting." Syrian activists say more than 20,000 people have died since the start of an uprising against Assad in March 2011.