UN leader Ban Ki-moon said that increased attacks by government forces on Syrian cities "violate" the UN Security Council's demand for an end to hostilities.
The UN secretary general strongly condemned new violence and indicated he believes President Bashar al-Assad is using an April 10 deadline to pull troops and heavy weapons away from cities as "an excuse" to step up killing.
Ban "deplores the assault by the Syrian authorities against innocent civilians, including women and children, despite the commitments by the government of Syria to cease all use of heavy weapons in population centers," said UN spokesman Martin Nesirky.
"The April 10 timeline to fulfill the government's implementation of its commitments, as endorsed by the Security Council, is not an excuse for continued killing," Ban added.
"Such actions violate the consensus position of the Security Council" which backed the peace plan drawn up by UN-Arab League envoy Kofi Annan and the April 10 deadline he agreed with Assad, he said, according to Nesirky.
The Security Council passed one statement backing Annan's peace plan and on Thursday agreed a second "presidential statement" formally endorsing the April 10 limit for Syrian troops and big guns to be pulled out of cities.
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Russia and China, which blocked two resolutions on Syria, have signed up to the new demands.
Syrian rights activists said however that since the new statement was released, at least 35 people were killed in government attacks on Friday on top of 77 killed on Thursday.
Ban said he was "gravely concerned" at the worsening humanitarian crisis in Syria. "The latest reports of growing numbers of refugee arrivals in neighboring countries are alarming," he said.
Turkey's Foreign Minister Ahmet Davutoglu informed Ban about a new surge in refugee arrivals in a telephone conversation on Thursday night, the UN spokesman said.sst
"The Syrian authorities remain fully accountable for grave violations of human rights and international humanitarian law. These must stop at once," the UN leader said.
Ban "demands that the government of Syria immediately and unconditionally cease all military actions against the Syrian people," the spokesman added.
"He reiterates that it is the responsibility of the Syrian authorities to now deliver on what they have promised, and to implement, fully and unconditionally, all the commitments under the six-point plan of joint special envoy Annan," the spokesman said.