UN Secretary General Ban Ki-moon urged the Security Council to "shoulder its responsibility" on the eve of Thursday's scheduled vote on possible sanctions against Bashar al-Assad's Syrian regime.
"Time is of the essence. The Syrian people have suffered for too long. The bloodshed must end now," Ban said, in a statement from his office, while UN envoys wrangled over details of the resolution ahead of the vote.
Ban on Wednesday also condemned the ongoing violence in Syria, in particular a bombing that killed three senior regime security officials and the reported use of heavy weapons by Syrian security forces against civilians in Damascus.
The statement said Ban "strongly condemns today's bomb attack at the National Security Headquarters in Damascus which resulted in significant casualties including the death and serious injury of government officials.
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"He is gravely concerned about reports of the continued use of heavy weapons by Syrian security forces, including in the Damascus area, against civilians, despite repeated government assurances such weapons would be withdrawn.
"The secretary general urges the Security Council to shoulder its responsibility and take collective and effective action on the basis of UN Charter obligations and in view of the seriousness of the situation."
The UN Security Council had been due to vote Wednesday on a Western-backed resolution that would have threatened Assad with international sanctions unless he withdrew his troops to barracks and implemented a UN-backed peace plan.
This went too far for Russia, a traditional friend of Damascus that is promoting a rival resolution with no provision for immediate sanctions, and the resolution appeared doomed to fall victim to Moscow's veto.
Kofi Annan, a former UN secretary general who is the envoy promoting a UN and Arab League peace plan, intervened to delay the vote to Thursday in the hope that diplomats will find a compromise deal.