De Mistura met with Syria’s primary opposition bloc, the Higher Negotiations Committee (HNC), on Monday and Wednesday. He met with delegates from Syrian President Bashar al-Assad’s government on Tuesday.
The meetings in Switzerland are part of a process outlined in a U.N. resolution last month that envisages an 18-month timetable for a political transition in Syria, including the drafting of a new constitution and elections.
Before de Mistura announced the temporary pause in negotiations, he had said the Geneva meetings were expected to last six months, with government and opposition delegations sitting in separate rooms and U.N. officials shuttling between them.
"The immediate priorities are a broad cease-fire, humanitarian aid, and halting the threat posed by the Islamic State in Iraq and the Levant (ISIL)," the U.N. had said in a statement.
Bashar al-Jaafari, Syria's ambassador to the U.N., blamed the pause on the HNC, telling reporters Wednesday that the "opposition's delegation came four days late and rejected to participate in the indirect negotiations with U.N. Special envoy for Syria."
The HNC had initially boycotted the talks, but changed course after being promised that their participation would lead to a cease-fire.
Farah Atassi, an HNC member, told Al Jazeera that opposition delegates were angry that Syria’s military and its ally Russia have continued to bomb civilian areas in opposition-held territory despite the negotiations.
"We are being targeted on the ground in Syria, politically and through the media," she said. "We came here to end the suffering of the Syrian people. Our presence here is a message to the international community: We will not back down from our demands. We call on the friends of Syria to pressure the Syrian government and Russia to end their airstrikes before any negotiations take place."
Citing reports from the Syrian Network for Human Rights (SNHR), a UK-based watchdog, the HNC accused Syrian and Russian forces of killing at least 300 civilians since the Geneva III conference began on Jan. 29.
The Syrian government launched a major offensive from the north of Aleppo and captured several strategically important towns on Monday. Syria's official news agency SANA reported that government forces on Wednesday cut off a supply route for rebels in Aleppo.
The UK-based Syrian Observatory for Human Rights said at least 16 airstrikes in Aleppo had been conducted since Wednesday morning. The monitoring group also said battles were taking place in the northern suburbs of Aleppo.
The HNC has condemned the Aleppo offensive, saying it shows that Assad's government is not committed to finding a peaceful resolution to the conflict.
HNC spokesman Riad Nassan Agha urged the United Nations on Tuesday to take measures in order to "deter Russia and the Syrian government from committing further crimes" against the Syrian people.
"These barbaric attacks are designed to put pressure on the opposition’s negotiating delegation and to make a mockery of the political process currently under way in Geneva and the international community that sponsors it," Agha said. "We came to Geneva to prove to the world that we are serious about finding a political solution at a time when the Assad regime, Russia and Iran escalate attacks on the Syrian people."
Like Russia, Iran is a backer of the Assad government.
The Geneva negotiations are meant to develop a "road map" to end the conflict, which has killed more than 250,000 Syrians.
The war has also displaced millions more, and sent hundreds of thousands fleeing as refugees to Europe.