UN-Arab League mediator Kofi Annan and Russia urged President Bashar al-Assad on Wednesday to speed up efforts to end the bloodletting in Syria, as regime forces overran another rebel city.
On the eve of the first anniversary of an anti-Assad revolt, the opposition suffered setbacks on both the military and political fronts, with its Syrian National Council (SNC) coalition hit by resignations.
International peace envoy Annan said he had received Assad's response to proposals he submitted in talks with the Syrian leader last week but had more questions that needed to be addressed without delay.
Annan still has "questions and is seeking answers," his spokesman said.
"But given the grave and tragic situation on the ground, everyone must realise that time is of the essence. As he said in the region, this crisis cannot be allowed to drag on," the spokesman added.
In Damascus, a foreign ministry spokesman said only that the authorities were "committed to cooperating in a positive manner with Annan's mission so long as there is goodwill to help Syria."
Annan is to brief the UN Security Council on his mission by videoconference from Geneva on Friday, diplomats in New York said.
One, who has been briefed on the answers already sent to Annan, said on condition of anonymity: "We had always expected there would be obfuscation, there would be delay, there would be questions."
On the ground, Noureddin al-Abdo, an activist in Idlib, confirmed the rebellious city in northwestern Syria had fallen on Tuesday night after a four-day assault by regime forces.
The outgunned, rebel "Free Syrian Army (FSA) has withdrawn and regime forces have stormed the entire city and are carrying out house-to-house searches," said Abdo, reached by telephone from Beirut.
The army launched a major offensive in Idlib province near the Turkish border on Saturday, bombarding the city and sweeping into rural areas in a bid to root out armed insurgents.
The Syrian Observatory for Human Rights reported fierce clashes between regime forces and rebel troops in the Jabal al-Zawiya district of the province, while at least 28 people had been killed across Syria on Wednesday.
Russia, which has been accused of weakening the international response to the crisis by blocking Security Council action, on Wednesday criticised Assad for his "big delay" in implementing reforms.
In a rare public rebuke from Moscow to the Syrian leader, Foreign Minister Sergei Lavrov accused Assad of "inertia" on the crisis that rights activists say has cost more than 8,500 lives in the past 12 months.
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"The side in the conflict in Syria on which we have influence is the government of Bashar al-Assad. Unfortunately, his actions, in practical terms, reflect our advice far from always and far from swiftly," Lavrov said.
"Yes, he has adopted useful laws to renew the system -- to make it more pluralistic than the one-party system that existed there -- but with a big delay," he told the lower house parliament, the State Duma.
On Tuesday, Assad issued a decree setting May 7 as the date for parliamentary elections under a new constitution adopted in February, with Washington dismissing the planned vote as "ridiculous."
The United States said Russia and China were moving closer to the rest of the international community in their positions on the crisis.
"You're now seeing public statements, both from Russia and from China, that are quite clearly saying that they are not interested in protecting Assad, that they are not interested in anything but something that ends the violence," State Department spokeswoman Victoria Nuland said.
After White House talks with British Prime Minister David Cameron, US President Barack Obama also called for world leaders to speak with one voice against the regime in Damascus.
"The best thing that we can do right now is to make sure that the international community continues to unify around the fact that what the Syrian regime is doing is unacceptable," Obama said.
UN chief Ban Ki-moon, who generally remains impartial in Security Council battles, said the council should adopt a resolution "immediately" demanding an end to the violence, saying it could change Assad's "political psychology."
"If he thinks he can weather this storm... he a serious misjudgment... He cannot continue like this. He has gone too deep, too far," Ban said.
The Syrian opposition, which is heavily outgunned on the battlefield by the regime and has called for its fighters to be armed in the defence of civilians, was hit on Wednesday by resignations from SNC ranks.
Haitham al-Maleh, Kamal al-Labwani and Catherine al-Talli announced on their Facebook pages that they were quitting the SNC, a coalition of Islamists, liberals and nationalists.
Labwani launched a scathing attack on the coalition formed last August in Istanbul to fight a political battle against Assad.
He said the trio had resigned because they did not want to be "accomplices to the massacre of the Syrian people through delaying, cheating, lies, one-upmanship and monopolisation of decision-making."
Saudi Arabia, which has been leading champion of arming the rebels, announced it was closing its Damascus embassy after withdrawing its ambassador last month.