The US has asked Syria to abide by a ceasefire agreement that is due to take effect this week as activists report the army is intensifying attacks against opposition fighters.
The warning came as government forces pounded more opposition strongholds on Saturday, activists said, killing nearly 130 people in an offensive that has sent thousands of refugees surging into Turkey before the UN-backed ceasefire aimed at staunching a year of bloodshed.
On Sunday, the main opposition umbrella group the Syrian National Council called for UN intervention after monitoring groups said 86 of those killed on Saturday were civilians.
Bashar al-Assad, the Syrian president, last week accepted a ceasefire agreement brokered by the UN-Arab League envoy Kofi Annan calling for government forces to withdraw from towns and villages by Tuesday, and for the government and opposition fighters to lay down their arms by 6am local time on Thursday.
The truce is meant to pave the way for negotiations between the government and the opposition over Syria's political future.
Country towns north of Aleppo have endured days of clashes and bombardment, prompting 3,000 civilians to flee over the Turkish border on Friday alone - about 10 times the daily number before Assad accepted Annan's plan 10 days ago.
Recep Tayyip Erdogan, the Turkish prime minister, said on Saturday the number of refugees entering Turkey was rising. Ankara fears an all-out war in Syria would unleash a flood of refugees.
"At the moment we have 24,000 Syrians who have entered Turkey. Of course this number is rising," Erdogan said before departing on a trip to China.
"We are taking measures for this, though we will not close the gates. The United Nations, however, has to toughen its stance," he said.
"In particular Kofi Annan has to hold firm. He announced a deadline of April 10. I believe that he should monitor the situation very closely."
The deaths on Saturday were reported from the cities of Homs, Hama and Aleppo by Syrian opposition groups and Al Jazeera sources.
The town of Taftanaz within the flashpoint province of Idlib also came under attack.
The Local Co-ordination Committees, an activist network, said on Saturday that at least 24 people were killed in the village of al-Latamneh in the suburbs of the city of Hama.
The UK-based Syrian Observatory for Human Rights put the al-Latamneh death toll at 27.
The group said most of those people were killed by shells fired as troops tried to storm al-Latamneh after clashes with defectors there over the past two days.
Farther to the south, at Qusayr in central Homs province, three civilians were killed, including a woman, a child and a defector from the police, said the UK-based activist group.
At least 77 people were killed across Syria on Thursday and 35 on Friday, most of them civilians, according to figures by the Syrian Observatory.
Ban Ki-moon, the UN secretary-general, sharply criticised the Syrian government on Friday for its latest attacks on civilians, saying that increased attacks by government forces on Syrian cities "violate" the UN Security Council's demand for an end to hostilities.
The UN press office said in a statement that the secretary general believes that Syrian 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] strongly condemns the latest escalation of violence," the statement said.
The UN statement continued: "He 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.
"The 10 April timeline to fulfill the government's implementation of its [ceasefire and troop withdrawal] commitments, as endorsed by the Security Council, is not an excuse for continued killing.
"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.
"The Syrian authorities remain fully accountable for grave violations of human rights and international humanitarian law," Ban's statement said. "These must stop at once."