More than 80 people, including two Western journalists, have been killed in shelling by Syrian forces in the city of Homs.
The barrages marked an intensification of a nearly three-week offensive.
US reporter Marie Colvin and French photographer Remi Ochlik were killed when the house in which they were staying after was hit by rockets.
The city has been under siege by President Bashar al-Assad's forces since 4 February.
Three other foreign journalists were wounded in the attack.
Ms Colvin was working for The Sunday Times newspaper in the country. She lost an eye when she suffered a shrapnel wound while working in Sri Lanka in 2001.
Mr Ochlik had photographed the Arab Spring for numerous publications.
Sunday Times editor John Witherow paid tribute today to Ms Colvin, saying she was an "extraordinary figure" who was "driven by a passion to cover wars in the belief that what she did mattered".
France has called on the Syrian government to immediately halt a military onslaught on Homs.
Russia has said it is working with the Syrian authorities, the opposition and regional powers to secure safe passage of humanitarian convoys.
Russia has asked the UN to request the Secretary-General send a representative to liaise with all sides in Syria on the safe passage of humanitarian convoys, Foreign Ministry spokesman Alexander Lukashevich said.
Mr Lukashevich said: "Our initiative is aimed at providing safety of humanitarian cargo deliveries, we are actively working with Syria and (countries) around it.
"We are working in this area with the Syrian leadership and representatives of the opposition, with the International Red Cross."
Separately an opposition group called on Syrians to boycott an upcoming referendum on a new constitution, saying it was an attempt by President Assad to cover up a crackdown.
Mr Assad has called the referendum for next Sunday and, if passed, multi-party elections will be held within 90 days.