Peace envoy Kofi Annan sought to shore up support from Moscow on Sunday for his efforts to end a year of fighting in Syria where 8,000 people have been killed in an anti-government uprising.
Meeting in Korea, U.S. President Barack Obama and Turkish Prime Minister Tayyip Erdogan discussed how to support the opposition with non-lethal aid.
Western and Arab states want Syrian President Bashar al-Assad to stand down but Russia, Syria's long-time ally, says the armed rebels must also cease fire and withdraw their forces.
As Annan, who is the United Nations and Arab League envoy, prepared to meet Russian President Dmitry Medvedev in Moscow, a U.S.-based human rights group accused Assad's forces of using human shields in their efforts to crush the rebellion.
In a statement ahead of the meeting, the Kremlin said it would be hard to enforce a halt to the violence until external armed and political support of the opposition is terminated.
Annan has drawn up a six-point peace plan, including demands for a ceasefire, the immediate withdrawal of heavy armour from residential areas and access for humanitarian assistance.
But, more than a year after the start of the uprising, the prospect of a negotiated peace seemed more remote than ever, with clashes reported around the country.
Moscow has accused the West of being too one-sided, arguing that outside support for rebels is fuelling the fighting in Syria, which hosts a Russian naval base.
But Russia and China, which have vetoed two U.N. resolutions critical of Damascus, did support a Security Council statement this week endorsing Annan's mission. The former U.N. chief is due to fly to China after Russia talks.
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.
White House deputy national security adviser Ben Rhodes, speaking to reporters after the two leaders met on the eve of a nuclear security summit, said Washington and Ankara were open to considering further non-lethal aid for the Syrian opposition at a Friends of Syria meeting in Turkey on April 1.
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, in a report on Sunday, quoted residents from Syria's northwestern province of Idlib saying Assad loyalists had forced them to march in front of advancing army offensives this month to retake control of areas that had fallen into the hands of the opposition.
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, it said.
HRW 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.
Syria says rebels have killed about 3,000 members of the security forces and blames the violence on terrorist gangs. The United Nations says the conflict has taken 8,000 lives.
The official Sana news agency said the bodies of 18 army martyrs, killed in various clashes, were buried at the weekend.
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.
The Syrian Observatory for Human Rights said three people were wounded on Sunday when mortars landed in the Homs district of Safsafa.
In the southern province of Deraa, birthplace of the revolt, government forces and rebels clashed on Sunday. In the eastern city of Deir al-Zor, Syrian troops conducted house-to-house raids in search of dissidents, SOHR said.
(Editing by Maria Golovnina and Giles Elgood)