* Syrian forces shell Homs at a rate of "one shell a minute"
* Advance team of six monitors due in Damascus
* Opposition rebels attack police station in Aleppo province
BEIRUT, April 15 (Reuters) - Syrian government forces shelled the city of Homs on Sunday and rebels attacked a police station in Aleppo province, resident opposition activists and a rights group said, in more violence four days after a ceasefire was meant to come into effect.
The fighting comes hours before an advance party of United Nations ceasefire monitors is due to arrive in Syria after Russia and China joined the rest of the Security Council on Saturday to authorise their deployment.
"Early this morning we saw a helicopter and a spotter plane fly overhead. Ten minutes later, there was heavy shelling," said Walid al-Fares, an activist living in Khalidiya, one of the neighbourhoods where mortars bombs have landed.
Another resident said government loyalists were using heavy machine guns to shoot into the area.
Rami Abdelrahman, head of the British-based Syrian Observatory for Human Rights, said shells were being fired at a rate of one a minute. He said there had also been overnight clashes in rural Aleppo.
"People said they heard explosions and shooting after rebels attacked a police station and then clashed with police," he said.
Syria blames the violence on "terrorists" seeking to topple President Bashar al-Assad and has repeatedly denied journalists access to the country, making it impossible to independently verify the reports.
Although violence has persisted throughout the ceasefire, there has been a significant drop in the daily death toll in fighting which has often killed more than 100 people a day.
On Saturday, 14 people were killed in the violence, Abdelrahman said and the state news agency SANA said "armed terrorists" killed five people in ambushes around the country.
ADVANCED MONITORING TEAM
The United Nations Security Council voted on Saturday to authorise the deployment of up to 30 unarmed observers in the first resolution on Syria the 15-nation council has managed to approve unanimously since the uprising erupted in March 2011.
Russia and China have previously blocked Western attempts to pass Security Council resolutions on Syria.
A spokesman for U.N.-Arab League envoy Kofi Annan said on Saturday an advance team of six monitors would arrive in Syria within 24 hours and deploy within 36 hours, with more to follow within days.
"I will make sure that this advance observer mission will be dispatched as soon as possible and try to make concrete proposals by the 18th of April for an official observer mission," U.N. Secretary-General Ban Ki-moon told United Nations radio separately in Geneva.
France's foreign ministry, while welcoming the U.N. vote, said it was now up to Syria to respect its commitments by withdrawing troops and heavy arms from populated areas.
"If this is not the case, it will be the responsibility of all the Security Council members to reflect on the measure that should be taken," it said in a statement.
VIOLENCE AND DIPLOMACY
The Security Council resolution condemned the "widespread violations of human rights by the Syrian authorities, as well as any human rights abuses by armed groups".
It said those responsible would be held accountable and called on "all parties, including the opposition, immediately to cease all armed violence in all its forms".
The text included a vague warning to Damascus, saying the council would "assess the implementation of this resolution and to consider further steps as appropriate".
"The Syrian government must ensure that the monitoring team has freedom of movement and access, and it must not obstruct communication between the monitors and headquarters," said British Foreign Secretary William Hague.
U.S. ambassador to the United Nations Susan Rice condemned what she said was Syria's "murderous rampage" over the last year. Asked if Syrian government shelling of Homs on Saturday was a violation of the ceasefire, Rice said: "Absolutely".
The U.N. estimates Assad's forces have killed more than 9,000 people in the uprising. Syrian authorities say foreign-backed militants have killed more than 2,500 soldiers and police.