UN Syria mediator Staffan de Mistura on Wednesday put the Geneva peace talks on hold until February 25, pointing to the level of violence happening on the ground.
Before the talks were halted, government forces dealt a major blow to rebels, cutting off vital supply lines to and from Aleppo, the last major urban centre in rebel hands.
De Mistura said military action had created an obstacle for the negotiations, which formally opened two days ago.
The indirect talks between the Syrian regime and opposition were often delayed by the parties' bickering and no observable progress had been made.
Syrians needed to see an improvement in humanitarian conditions in tandem with political talks, de Mistura said.
"Since I didn't see that ... I have to be honest with myself and say it's time for a pause, only a pause," he said.
"I have indicated from the first day I won't talk for the sake of talking. I therefore have taken the decision to bring a temporary pause. It is not the end or the failure of the talks," he said.
The head of the Saudi-based opposition Higher Negotiations Committee, Riad Hijab, said during a press conference following de Mistura's decision that "we believe whoever wants a political solution will not resort to escalation (in reference to the regime) ... we know that this regime does not want a political solution."
He said a ceasefire cannot be achieved as long as Bashar al-Assad is still in power.
The opposition delegation would leave Geneva on Thursday "and will not return until all humanitarian issues are achieved," Hijab said.
He called on the world powers to exert pressure on the regime to stop its crimes against the Syrian people.
US Secretary of State John Kerry said while the talks are paused "the world needs to push in one direction - toward stopping the oppression and suffering of the Syrian people and ending, not prolonging, this conflict."
The assault by Syrian regime forces - enabled by Russian airstrikes - against opposition-held areas, and the besieging of civilians signaled an intention to seek a military solution rather than a political one, Kerry said.
The head of the regime delegation, Bashar al Jaafari, was quoted by Syria's state media as saying that de Mistura's decision to pause the negotiations was a "diplomatic one to cover the decision by the opposition who wanted to withdraw from the talks."
In Syria, troops and allied militiamen had "completely cut the supply routes of the terrorist organizations" north of Aleppo and lifted the siege on two Shiite villages that had been surrounded by rebels for over three years, state news agency SANA reported.
The Syrian Observatory for Human Rights, an opposition monitoring group, confirmed that the route between Aleppo and the Turkish border had been blocked after advances by government forces backed by intense Russian airstrikes.
The development is a significant victory for government forces, who have been trying for over two years to encircle Aleppo. Other rebel-held routes to the northern city remain open.
Russian Foreign Minister Sergei Lavrov made it clear earlier that Moscow had no immediate intention of pulling back on its military support for the regime.
Speaking to reporters in Muscat, Oman, Lavrov singled out access to the Turkish border as a key issue.
"The key point for the ceasefire to work is a task of blocking illegal trafficking across the Turkish-Syrian border, which supports the militants," he said in comments reported by Russian state news agency TASS.
Lavrov said the aim of Russian operations was "to defeat the Islamic State and Jabhat al-Nusra terrorist groups."
Al-Nusra, the Syrian branch of al-Qaeda, is a key component of the main rebel alliances on the ground in northern Syria. Days ago it sent a large column of fighters to reinforce rebels in Aleppo.
The Geneva talks, officially backed by Russia, the United States and other countries variously allied with al-Assad's government and the rebels, are aimed at finding a political solution to the five-year conflict that has devastated Syria and driven half its population from their homes.
The opposition insists on humanitarian measures and an end to Russian and Syrian airstrikes as a first step toward a resolution to the conflict, while the government says the focus must be on combatting terrorism.
In Saudi Arabia, German Foreign Minister Frank-Walter Steinmeier said al-Assad's government is responsible for the halt to UN-brokered Syria peace talks in Geneva.
"It became increasingly clear in the last few days how the Geneva talks were affected by the Syrian military's offensive in Aleppo, and the unwillingness of the al-Assad regime to allow humanitarian access to the besieged towns and villages," he said.