Russia offered full support for peace envoy Kofi Annan's efforts to end fighting in Syria on Sunday but said his mission would need more time as forces loyal to President Bashar al-Assad attacked Homs and other rebel strongholds.

Moscow also reiterated its stance that foreign support for the Syrian opposition was the main obstacle to peace, while U.S. President Barack Obama and Turkish Prime Minister Tayyip Erdogan discussed how to get non-lethal aid to the opposition.

Western and Arab states have urged Assad to step aside to end violence which the U.N. says has cost 8,000 lives. Russia, a close ally of Assad, said he is ready to talk to his foes on reform and it is the rebels who must be pressed to negotiate.

With the Syrian army on the offensive around the country and the opposition fearing Assad would use any talks to strengthen his forces' position and crack down harder, the prospect of a negotiated peace seemed more remote than ever.

A U.S.-based human rights group accused Assad's forces on Sunday of using human shields in their efforts to crush the rebellion, which began more than a year ago.

Syrian government forces have endangered local residents by forcing them to march in front of the army during recent arrest operations, troop movements, and attacks on towns and villages in northern Syria, Human Rights Watch said, quoting residents from Syria's northwestern province of Idlib

Russian President Dmitry Medvedev, who will join Obama for a nuclear security summit in South Korea on Monday, told U.N.-Arab League Syria envoy Kofi Annan in Moscow he appreciated his efforts to end the violence.

This may be the last chance for Syria to avoid a long-lasting and bloody civil war. Therefore we will offer you our full support at any level and in various ways in those areas, of course, in which Russia is capable of providing support.

It was not clear whether Moscow would use its influence with Assad to advance Annan's six-point peace plan, which includes demands for a ceasefire, the immediate withdrawal of heavy armour from residential areas and access for humanitarian aid.

Syria has an opportunity today to work with me and this mediation process to put an end to the conflict, to the fighting, allow access to those in need of humanitarian assistance as well as embark on a political process that will lead to a peaceful settlement, Annan said after the talks.

Russian Deputy Foreign Minister Gennady Gatilov said Annan's mission must be given more time before the Security Council considers further action. A Security Council statement this week threatened Syria with unspecified further steps if it failed to comply with Annan's plan.

There are no deadlines, we need to see how the situation develops, the Interfax news agency quoted Gatilov as saying.

Moscow has accused the West of being too one-sided, arguing that outside support for rebels, which it says is both political and military in some cases, is fuelling the fighting in Syria, which hosts a Russian naval base.

A Russian foreign ministry statement said Annan should work hard with both government and opposition in Syria and for his mission to succeed it needed full international support.

This entails non-interference in Syria's internal affairs and inadmissibility of supporting one side in the conflict, it said, quoting Foreign Minister Sergei Lavrov.

Russia and China have vetoed two U.N. resolutions critical of Damascus, but did support the Security Council statement this week endorsing Annan's mission. The former U.N. chief is due to fly to China after his talks in Russia.

Largely to ensure Russian support, the statement included no firm deadline for implementation of its demands, potentially allowing Assad to play for time. It also included no direct call for Assad to cede power, which Russia would also have opposed.

In the Korean capital Seoul, Obama and Erdogan discussed providing medical supplies and communications support to the Syrian opposition but there was no talk of providing lethal aid for rebel forces.

We worked on a common agenda in terms of how we can support both humanitarian efforts ... (and) the efforts of Kofi Annan to bring about much needed change (in Syria), Obama said after his meeting with Erdogan, a sharp critic of Assad.

New York-based Human Rights Watch published videos, obtained from opposition activists, in which people in civilian clothes walk in front of several armed soldiers and infantry fighting vehicles. Activists say the army had compelled the men to walk in front to protect the soldiers.

The statement said that residents reported government forces placing children on tanks and inside security buses.

The Syrian army's use of human shields is yet another reason why the UN Security Council should refer Syria to the International Criminal Court, said Ole Solvang, a HRW emergencies researcher.

It was impossible to verify reports independently because Syrian authorities have prevented foreign journalists and human rights workers from entering affected areas.

HEAVY SHELLING

Syria says rebels have killed about 3,000 members of the security forces and blames the violence on terrorist gangs.

Syrian troops have repeatedly targeted Homs, Syria's third largest city, and said last month they had regained control of Baba Amr, a neighbourhood held by rebels for several months.

However, a surge in violence in other neighbourhoods this week suggested the army was struggling to keep control.

Waleed al-Faris, an opposition activist from Homs, told Reuters that Sunday's shelling, using tank and mortar fire, was the worst he had seen.

There are ten dead and hundreds wounded, he said. I have not experienced shelling this heavy since Baba Amr.

In the southern province of Deraa, birthplace of the revolt, government forces and rebels clashed on Sunday.

Thousands of soldiers and over a hundred military vehicles are attempting to enter the area of Lahat in Deraa province today, but they are clashing with rebels, said Rami Abdelrahman, head of the Syrian Observatory for Human Rights, adding that at least five soldiers and three rebels had been killed.

The Syrian state news agency SANA said six terrorists had been killed on during dawn raid on a hideout in Deraa.

The SOHR said 27 were killed around Syria on Sunday, 15 of them civilians, during heavy shelling in the central city of Homs and northwestern province of Idlib.

In the eastern city of Deir al-Zor, Syrian troops conducted house-to-house raids in search of dissidents, SOHR said.

(Editing by Philippa Fletcher and Giles Elgood)