* Deadline expired at 0300 GMT, activists report calm
* U.N. observer planning team leaves Syria, no explanation given
* Activists say no sign of army withdrawal from cities
By Mariam Karouny and Dominic Evans
BEIRUT, April 12 A U.N.-backed ceasefire aimed at halting more than a year of bloodshed in Syria appeared to be holding early on Thursday but activists saw no sign that President Bashar al-Assad was pulling his forces out of restive cities.
The flashpoint provinces of Homs, Hama and Idlib, which have been under sustained shelling by Assad's forces over the last week, were calm after the 6 a.m. (0300 GMT) ceasefire deadline. An activist in Damascus said the capital was also quiet.
"More than two hours have passed and it's completely quiet across the country," said Rami Abdulrahman, director of the British-based Syrian Observatory for Human Rights (SOHR).
The Syrian government bars access to most independent media.
Fighting had raged in the countdown to Thursday's deadline, fuelling widespread doubts that Syria would comply with international envoy Kofi Annan's ceasefire plan.
World leaders, grappling with fundamental disagreements particularly between the West and Russia over how to deal with the crisis, are monitoring events on the ground closely.
"It was a bloody night - there was heavy shelling on the city of Homs, but now it is calm and there is no shooting," said an activist who called himself Abu Rami, speaking from Syria's third largest city which has endured some of the worst violence.
But, like activists in other towns, he said there was no indication that Syrian army forces were withdrawing from Homs in accordance with the ceasefire agreement. "There are no signs of a pull back - the tanks, snipers and armed forces are still visible across the city," he said.
The Syrian Observatory reported explosions in Zabadani, a town close to the Lebanese border, shortly after the deadline. But a resident contacted by Reuters said the town was quiet.
Syria's Defence Ministry said on Wednesday it would halt operations on Thursday morning but made no mention of an army pullback from cities and said it would confront "any assault" by armed groups
A spokesman for Annan said on Wednesday night an advance planning team negotiating how U.N. observers would monitor the accord had left Damascus after a week of talks.
He had no further comment on any progress reached by the team led by Norwegian Major-General Robert Mood, so it was not clear whether the deployment had been agreed or called off.
Annan is due to brief the U.N. Security Council on developments in Syria at 1400 GMT.
Insurgents, who lack a clearly coordinated command structure, said they would stop shooting if Assad's forces withdrew and observed the truce.
"The Defence Ministry announcement is a detour on Annan's plan which clearly says he should pull back the tanks and end violence. We will wait until tomorrow and see. We will not act before tomorrow," Qassem Saad al-Deen, Free Syrian Army spokesman inside Syria, told Reuters on Wednesday.
At least 12 people were killed on Wednesday, activists said.
Western powers have scorned Assad's truce pledges and largely sympathise with the revolt against him, but so far lack an effective policy to curb the bloodshed, given their own aversion to military intervention and the resistance of Russia and China to any U.N. Security Council action against Assad.
Quoting from a letter to Annan from the Syrian foreign ministry, Annan's spokesman Ahmad Fawzi said the government had undertaken "to cease all military fighting throughout Syrian territory as of 6 a.m. (0300 GMT) tomorrow, Thursday, 12 April, 2012, while reserving the right to respond proportionately to any attacks carried out by armed terrorist groups against civilians, government forces or public and private property".
He also stressed that troops should pull back.
Syria's ambassador to the United Nations, Bashar Ja'afari, said in a U.S. television interview that his government was "on board" with the peace plan. But government forces would remain on alert to "counterattacks" and that "legally speaking, there is a big difference between declaring a ceasefire and putting an end to the violence."
Activist videos posted on YouTube on Wednesday showed a shopping mall engulfed in flames after it was hit in bombardment of the Juret al-Shayah district of Homs and bombs crashing into the city's Khalidiya district. The videos could not be verified and the Syrian government bars most independent media.
"Fighting is still raging as we speak, reflecting what has been an intensification of the violence that the Syrian government has pursued since April 1 when it committed to cease all hostile actions by yesterday," U.S. Ambassador to the United Nations Susan Rice said in New York on Wednesday afternoon.
"Its commitments, therefore, have little if any credibility given that track record," she told reporters.
The exile opposition Syrian National Council (SNC) called for an international ultimatum to Assad if he failed to respect the ceasefire.
"We would like to see a unanimous decision by members of the Security Council that sends an ultimatum to the regime with a deadline that is not too far down the road that says on such and such a date enforcement measures will intervene," SNC spokeswoman Basma Kodmani said on Wednesday.
U.N. action would need the support of Russia and China, which have blocked previous Security Council draft resolutions on Syria, citing concerns about a Libya-style intervention that would breach Syrian sovereignty.
Turkey, hosting nearly 25,000 Syrian refugees, has been particularly alarmed and Prime Minister Tayyip Erdogan raised the prospect of engaging its NATO allies to protect its borders after shooting by Syrian troops hit people in a refugee camp.
"NATO has responsibilities to do with Turkey's borders," he was quoted as saying by Hurriyet newspaper on Thursday, citing Article 5 of the North Atlantic defence pact which allows for a common response to an attack on the territory of a member state.
Across Turkey's border with Syria, fighting raged on Wednesday evening, refugees at the border town of Kilis said.
Ankara has urged the Security Council to adopt a resolution that would protect the Syrian people, saying Damascus had not kept its troop withdrawal pledge and had increased the violence.
Annan, at a news conference in Tehran on Wednesday, urged Iran to help resolve the violence and warned of "unimaginable consequences" if it worsened further.
Iranian President Mahmoud Ahmadinejad, quoted by state news agency IRNA, urged an end to violence but accused the NATO powers of expansionist ambitions in the Middle East and said Tehran's Syrian ally should not be put under pressure:
"NATO is not ashamed to say it wants to dominate the region and is trying to extend its domination eastward," IRNA said, quoting Ahmadinejad as insisting: "The implementation of any plan in Syria should be free of pressures and interference and all violence in that country should be stopped."
Assad's forces have killed more than 9,000 people in the past year, according to a U.N. estimate. Damascus says rebels have killed more than 2,600 soldiers and security personnel.