Staffan de Mistura admitted there had been a lack of progress but said that the negotiations had not failed.
The talks opened just two days ago and are due to resume again on 25 February.
The move came as the Syrian government claimed a major blow against the opposition by cutting a key supply route to the rebel-held city of Aleppo.
Syrian state television reported that government forces had broken the siege of Nubul and Zahraa, two towns north-west of Aleppo.
Opposition attendees at the talks in Geneva had already been angered at negotiations beginning amid ongoing offensives by the Syrian government backed by Russia.
Basma Kodmani, a member of the umbrella group representing opposition factions, the High Negotiations Committee, said encircling Aleppo sent the message that "there is nothing to negotiate - Just go home".
On the talks, Mr de Mistura admitted "there's more work to be done".
"It is not the end and it is not the failure of the talks," he said.
"Why? They came and they stayed. Both sides insisted on the fact that they are interested in having a political process started."
The head of the Syrian delegation accused the opposition of wanting to withdraw from the talks under pressure from Saudi Arabia, Qatar and Turkey, Reuters reported.
Bashar Ja'afari said the UN's announcement was a way of avoiding such a walkout.
More than 250,000 people have died in almost five years of war in Syria.
Eleven million others have fled their homes as forces loyal to President Bashar al-Assad and those opposed to his rule battle each other, as well as the Islamic State group.
A donor's conference opens in London on Thursday aiming to raise billions in new funding for those affected.
The UN has suspended peace talks aimed at ending Syria's five-year civil war, the organisation's special envoy has said.