A week of peace talks ended in Geneva on Friday with no concrete progress and no immediate commitment from Assad's envoys to return on Feb. 10 for more meetings with the Western-backed opposition as suggested by mediator Lakhdar Brahimi.
U.N. Secretary-General Ban Ki-moon told a conference of global security officials in Munich that he urged Russian Foreign Minister Sergey Lavrov and U.S. Secretary of State John Kerry at a meeting on the sidelines ''to use their influence to ensure the talks proceed as scheduled on Feb. 10.''
The U.S. has insisted that Assad cannot be part of a transitional government and has lost his legitimacy to lead, while Russia has been a key ally of Assad's government.
Ban urged the warring parties to ''come back with more sense of earnestness as well as seriousness and urgency.'' Specifically, he called on ''both sides and the government in particular to allow the unfettered access required under international humanitarian law.''
An agreement to allow aid convoys into the central Syrian city of Homs has remained stalled, with the government and opposition accusing each other of holding up the aid delivery into the city, which has been under siege for nearly two years.
Lavrov insisted that ''Russia can do nothing alone'' and urged the U.S. and others to exert their influence on the Syrian opposition.
Lavrov said the humanitarian situation in Syria is ''outrageous'' but insisted that ''we've got to be realistic,'' arguing that the Syrian government is willing to deliver aid to Homs and deliveries to other cities, such as Aleppo and suburbs of Damascus, should also be an issue.
''I can assure you that we are putting daily pressure on the Syrian government,'' Lavrov said during a panel discussion at the conference. ''It is a very difficult situation and to try to convince the government, which is waging a war, to make some gestures - this is a very difficult task.''
Kerry raised concern over the humanitarian situation, especially in the besieged areas such as Homs, when he met with Lavrov Friday night. He also insisted anew that the creation by mutual consent of a transitional governing body must be a primary focus in the next round of talks, according to a senior State Department official who wasn't authorized to discuss the private meeting by name.
The meeting discussed the need for the opposition to expand its delegation, the official said.
The opposition delegation does not control armed groups inside Syria, including al-Qaida-backed militants, who do not feel bound by agreements reached in the talks.
Brahimi has struggled to find positive words about the outcome of the first round of talks, but Lavrov welcomed ''the modest but important results'' and said he hoped they will be ''deepened and expanded'' in the next round.
Ban said that ''it is hard going but we have made a start.''
''The parties may still be fighting but now they are also talking - this is the only hope for a political solution,'' he said.