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 suggested 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 deeply divided 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.

Syrian activists said 27 people had been killed on Sunday, including 15 civilians, and a U.S.-based human rights group accused Assad's forces 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 Annan, the Syria envoy for the United Nations and Arab League, 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 increase pressure on Assad to comply with Annan's peace plan, which includes demands for a ceasefire, the immediate withdrawal of heavy armour from residential areas and access for humanitarian aid.

Russia has shielded Assad from U.N. Security Council condemnation by vetoing two Western-backed resolutions over the bloodshed, but has criticised the Syrian leader recently and approved a Security Council statement this week endorsing Annan's mission.

The former U.N. chief is due to fly to China, which joined Russia in the vetoes, after his talks in Russia.

"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 at the start of his talks with Medvedev at a Moscow airport.

Russian Deputy Foreign Minister Gennady Gatilov said Annan's mission must be given more time before the Security Council considers further action. The 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.

Annan's spokesman, Ahmad Fawzi, said he had "very candid and comprehensive discussions" in Moscow. Annan was "grateful for Russia's firm support for his mediation efforts in order to reach a swift and peaceful solution to this bloody conflict" and asked Russia to continue providing support.

Moscow has accused the West and Gulf Arab nations of being too one-sided, arguing that foreign political support for the opposition and contraband weapons supplies to rebels fuel the fighting in Syria, which hosts a Russian naval base.

Western and Arab leaders are due to meet in Istanbul next week to back the revolt against Assad and the Arab League and Turkey were pressing the opposition to unite beforehand.

Russian Foreign Minister Sergei Lavrov urged Annan to work with both government and opposition and said his mission needed full international support, the Foreign Ministry said.

"This entails non-interference in Syria's internal affairs and inadmissibility of supporting one side in the conflict," the ministry said in a statement after their meeting.

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 to the rebels.

"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.