At least 27 Syrian soldiers, rebels and civilians were killed in violence on Friday, opposition activists said, four days before a troop pullback agreed by President Bashar al-Assad as part of a U.N.-backed peace plan.

The Free Syrian Army (FSA) said it had met a delegation from peace envoy Kofi Annan this week and confirmed its fighters would stop shooting if Assad withdraws his tanks and troops to barracks before a ceasefire deadline next Thursday.

"Talks were held and the FSA said if the regime commits to the plan and withdraws from the cities and returns to its original barracks then we are committed to the plan," Colonel Riad al-Asaad told Reuters. He declined to give further details.

The plan calls for a troop withdrawal by April 10 and a ceasefire by April 12. Assad told Annan two weeks ago he had accepted the terms. The Annan plan does not stipulate a withdrawal to barracks. It says the army must "begin pullback of military concentrations in and around population centres".

But on Friday at the United Nations, Secretary-General Ban Ki-moon sharply criticized the Syrian government for the persistent attacks on civilians and demanded it keep its pledge to halt all military operations.

He suggested the latest killings were tantamount to a violation of U.N. Security Council demands that Damascus move toward a ceasefire.

"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," the statement from Ban's press office said.

"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."

The Syrian Observatory for Human Rights said shelling had killed at least 10 people, including four rebel fighters, in the flashpoint central city of Homs. Two soldiers died in separate clashes and one person was killed in the town of Douma, it said.

The British-based Observatory, which has a network of contacts in Syria, also reported seven civilians and four soldiers killed in clashes and bombardments in Anadan, north of Aleppo. Three people were killed in Hama, it said.

Annan has said the government and opposition must stop fighting at 6 a.m. (0300 GMT) on April 12, if Damascus meets its deadline 48 hours earlier to pull back troops from cities and cease using heavy weapons.

Assad's opponents have accused the Syrian military of using the run-up to the ceasefire to intensify assaults. Syria has now charged insurgents with doing the same.

"In recent days terrorist acts committed by armed groups in Syria have escalated, especially since an understanding was reached on Kofi Annan's plan," Syria said in a letter to the U.N. released on Friday.

"The international community and the Security Council must take the necessary measures to prevent and stop the funding of any terrorist activities against Syria," it said.

Syrian forces were laying mines near the border with Turkey to try to block a flow of refugees and supplies for insurgents, rebel activists and a Turkish official said.

"The Syrians have been mining the border, especially the southern Idlib part which has been restricting the flow of refugees," said the official, who declined to give his name.

Turkey said there were now 23,835 Syrian refugees on its territory of whom 2,800 arrived on Thursday alone, more than double the highest previous one-day total.