Geneva: The UN was struggling to keep troubled Syria peace negotiations on track, as the opposition suspended its "formal participation" in the talks in protest at escalating violence on the ground.
Russian President Vladimir Putin and his US counterpart Barack Obama meanwhile agreed to help strengthen a fragile ceasefire in Syria, seen as essential to paving the way towards a lasting deal to end the bloodshed.
With fighting surging around Syria's second city Aleppo and negotiations in Geneva stalled over the fate of President Bashar al-Assad, the opposition said they had had enough.
"Since these talks began in Geneva, the Assad regime has worsened the situation on the ground," Salem al-Meslet of the main opposition High Negotiations Committee said in a statement.
The group said the talks must be placed on "brief hold in order to end the Assad regime's truce violations", accusing the regime of more than 2,000 violations of the ceasefire since it began on 27 February.
"The HNC remains fully committed to the political process and establishing peace through diplomacy," Meslet said. "We remain in Geneva ready to engage in serious talks."
HNC coordinator Riad Hijab said earlier it was "unacceptable" for the negotiations to continue while Assad's regime continues to "bombard and starve civilians" in Syria.
The regime responded by accusing Saudi Arabia, Turkey and Qatar - all backers of the opposition - of seeking to derail the negotiations.
"The Saudi, Turkish and Qatari sponsors do not want to stop the bloodbath in Syria and do not want a political solution in Syria," the regime's lead negotiator in Geneva, Bashar al-Jaafari, said in an interview late yesterday with Lebanese channel Al-Mayadeen.
Jaafari added that the opposition and the countries that support it are "annoyed because of the progress being made by the Syrian army on the ground".
But UN peace envoy Staffan de Mistura insisted the ongoing round of indirect discussions, which began on April 13, would continue through the week as planned.
The HNC may continue to meet UN staff at their hotel "to pursue technical discussions", including on a political transition in Syria, de Mistura said.
The UN envoy stressed that the indirect talks format, which has seen the HNC and Assad's representatives meet separately with UN mediators, created flexibility to continue the discussions.
But the opposition's announcement is a clear blow to the already precarious negotiations aimed at achieving political transition, a new constitution, and presidential and parliamentary elections by September 2017.
The landmark ceasefire negotiated by Russia and the US in February dramatically curtailed violence across much of Syria, raising hopes that a lasting deal could be struck to end the five-year civil war.