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 February 27.
"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 Monday 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".
- Clear blow -
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.
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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.
But the spread of fighting around Aleppo and the regime's apparent unwillingness to hold concrete talks on forming a new government has led the opposition to question Assad's commitment to a political solution to a conflict that has killed more than 270,000 people.
Areas in Syria controlled by jihadists like the Islamic State (IS) group and Al-Qaeda-linked Al-Nusra Front are exempt from the ceasefire, but the renewed Aleppo clashes are drawing in other rebel groups and straining the truce.
The Syrian Observatory for Human Rights said 22 civilians were killed on both sides over the weekend in Aleppo city -- one of the highest single tolls since the truce began.
Medical charity Medecins Sans Frontieres (MSF) warned the situation was "critical" for more than 100,000 people trapped by fighting in the Azaz district of Aleppo governorate.
Piling further pressure on the ceasefire, 10 rebel groups vowed Monday to strike back against alleged violations of the truce.
- US, Russia try to boost ceasefire -
Despite the surging violence on the ground, De Mistura insisted "the cessation of hostilities is holding," but acknowledged: "We are concerned."
Putin and Obama meanwhile agreed during a phone call Monday to help strengthen the ceasefire, according to the Kremlin.
Both leaders had stressed the "significance" of the Geneva talks and agreed that their security services and defence ministries would ramp up cooperation over Syria, it said.
"With this end in view additional measures on how to quickly react to existing ceasefire violations will be worked out," the Kremlin added.
French President Francois Hollande also said everything must be done to maintain the truce.
"If negotiations do not resume, if fighting does, there would again be the worst fears for civilians (and) consequences in terms of refugees," he said.
Regardless of whether the ceasefire is strengthened, the Geneva talks will likely continue stumbling on the tricky issue of Assad's fate.
The HNC insists that any peace deal must include Assad's departure from power. Damascus has so far said the president's future is off limits.
"The gap is clearly wide," de Mistura admitted.