"We are deeply concerned by events in Syria, particularly the recent incidents on the border with our ally Turkey," NATO spokeswoman Carmen Romero told AFP.

Turkish Prime Minister Recep Tayyip Erdogan said Wednesday that he may invoke NATO's Article 5, which says an attack on a member constitutes an attack on all allies, following the incident.

"We take our responsibility to protect NATO allies extremely seriously," Romero said.

"We are monitoring the situation very closely and will continue to do so."

NATO Secretary General Anders Fogh Rasmussen insisted last week that the alliance has "no intention whatsoever to intervene in Syria" as he pressed for a diplomatic solution to a brutal crackdown on dissidents.

But Erdogan was quoted by Today's Zaman newspaper on Wednesday as saying that "NATO has a responsibility to protect Turkish borders."

The Turkish government reacted furiously after shots fired from Syria wounded four Syrians and two Turkish staff working at a refugee camp in Turkey on Monday.

The NATO spokeswoman said the alliance fully supports the peace plan put forward by UN-Arab League envoy Kofi Annan.

"Now the whole international community must make every effort to ensure the plan is put into effect," Romero said. "The whole world is watching Syria -- and the whole world wants to see an end to this intolerable violence."

The UN-backed ceasefire to end 13 months of bloodshed in Syria appeared to be holding after coming into force at daybreak Thursday, despite doubts about the regime's compliance with the peace plan.

A UN-backed ceasefire to end 13 months of bloodshed in Syria appeared to be holding after coming into force at daybreak, but doubts remained about the compliance of President Bashar al-Assad's government with the peace plan.

"China welcomes the Syrian government's decision, which will help ease tensions in Syria," said ministry spokesman Liu Weimin in a statement.

"This will help ease the tense situation in Syria, and is an important step towards a political solution."

China's Foreign Minister Yang Jiechi had earlier written to his Syrian counterpart asking him to "actively evaluate a decision for a ceasefire and troop withdrawal," the statement said.

Annan, serving as a peace envoy for the United Nations and the Arab League, said earlier he had received a letter from Syria promising to "cease all military fighting throughout Syrian territory as of 6:00 am".

But the letter reserved the right "to respond proportionately to any attacks carried out by armed terrorist groups", a phrase Assad's government uses to describe opposition forces.

Monitors said the military unleashed a lethal offensive against protest centres hours before that deadline expired, killing 25 civilians on Wednesday, including 10 in the rebel stronghold of Rastan.

They also said there were no signs of troops withdrawing from cities, after the regime in Damascus broke a commitment to pull back forces from population centres by Tuesday under the agreement brokered by Annan, a former UN chief.

Beijing gave its seal of approval to the Annan deal late last month after drawing international criticism for vetoing two UN Security Council resolutions on the crisis in Syria.

On Thursday, British Prime Minister David Cameron urged Russia and China to join the international community and help "tighten the noose" on the Syrian regime over its deadly crackdown on protests.

His comments come after US President Barack Obama and German Chancellor Angela Merkel agreed in a telephone call that the UN Security Council, which backed Annan's plan, needed to "take more resolute action" on Syria.