(CNN) -- United Nations-Arab League special envoy Kofi Annan said Monday the ongoing crisis in Syria cannot not be allowed to "drag on indefinitely," but resisted setting any sort of timetable.
He addressed reporters in Moscow, where a day earlier he met with the Russian president and foreign minister.
"I think it is urgent to move ahead," Annan said, according to a transcript of his remarks "But it is not practical to put out timetables and timelines when you haven't gotten an agreement from the parties."
Annan has offered a six-point plan -- supported by the U.N. Security Council -- as a way to halt the violence. The Syrian government has formally responded to the proposal, a spokesman for Annan said Monday.
The spokesman did not provide details about the Syrian government's response, but said Annan was studying it and would respond shortly.
Annan is headed next to China.
Russia and China have so far blocked Security Council attempts to pass resolutions condemning the government of Syrian President Bashar al-Assad. The two countries say they want the violence to stop but argue that previous resolutions were not evenhanded.
"This cannot be allowed to drag on indefinitely. As I have told the parties on the ground, they cannot resist the transformational winds that are blowing. They have to accept that reforms have to come, change has to come, and that is the only way to deal with the situation," Annan said.
As top-level negotiations continued, at least 59 people were killed as government forces shelled buildings and snipers fired on civilians in Syria on Monday, opposition activists said.
Heavy shelling rained on Homs, which has been a hotbed of anti-government sentiment, opposition activists said. At least 33 people were killed there, according to the opposition Local Coordination Committees of Syria.
The dead include four young men gunned down when residents tried to retrieve two bodies, the opposition Syrian Network for Human Rights said.
Also Monday, the Avaaz opposition group said Syrian authorities had arrested an activist who played a key role in delivering humanitarian aid into Homs and other besieged cities, in addition to helping foreign journalists enter Syria to report on the violence.
Security forces ambushed Syrian activist Jassim Khaled Diab on Saturday while he was trying to evacuate an injured person across the border into Lebanon, Avaaz said.
His reported detention calls into question the future reliability of a route that smugglers who oppose the Syrian government have used to deliver aid to besieged cities, such as Homs.
The state-run Syrian Arab News Agency reported that Homs residents thanked Syrian troops Sunday "for restoring security and stability to their neighborhood and saving them from the horror of the armed terrorist groups."
On Monday, SANA said authorities had killed and wounded members of an "armed terrorist group" that were trying to infiltrate Syria from Turkey.
The Syrian government routinely blames the vaguely defined "armed terrorist groups" for violence in the country, while most reports from inside Syria indicate the government is slaughtering civilians in an attempt to wipe out dissidents.
CNN cannot independently confirm reports from inside Syria because the government severely restricts access by international journalists.
Also on Monday, Turkey announced it had suspended operations at its embassy in Damascus. Turkey's consulate in Aleppo, however, is still functioning, Turkish Foreign Ministry spokesman Selcuk Unal said.
Norway also announced it had closed its embassy in Damascus, citing security concerns.
While reports of terror and carnage mounted inside Syria, foreign-based opposition members met in Turkey, aiming to form a unified front before a global conference on the Syrian crisis.
Meetings in Istanbul between exile opposition groups aim to find common ground before the upcoming Friends of Syria conference, which will bring representatives of many countries to Turkey on Sunday. It will be the group's second meeting.
U.S. Secretary of State Hillary Clinton is expected to be in Istanbul this weekend to attend the conference, State Department spokeswoman Victoria Nuland said in a statement.
"Most of the groups realize they have to unite to have a road map to get the support of the international community," said Omar Shawaf, a member of the Syrian National Council, an opposition coalition.
Participants said they hoped informal talks between opposition groups Monday would pave the way for an agreement later in the week.
The goal of the conference is "for all forces and parties of the opposition not to be a union, but at least to have a united purpose," said Ammar Qurabi, leader of the National Change Current opposition group.
A key goal of the meeting will be getting Kurdish groups to support the opposition, he said. Kurds make up about 10% of Syria's population, but have not played a major role in recent anti-government protests.
"I think the Kurds could be a kingmaker. ... They're not acting in harmony with the Syrian opposition. They're not protesting in large numbers. Most of them stay in their homes," said Ufuk Ulutas, a Middle East expert at Turkey's Foundation for Political, Economic and Social Research. "They're waiting for something."
Observers of the Syrian crisis have bemoaned a lack of unity among Syrian opposition groups as they struggle to topple President al-Assad.
National Change Current is not a member of the Syrian National Council, which has gained recognition from the United States, France and other countries as a legitimate representative of the Syrian opposition.
The SNC was the only opposition body represented at the first Friends of Syria conference, held last month in Tunisia. Dozens of international leaders at the summit condemned the Syrian regime's deadly crackdown on peaceful protesters while expressing support for the Syrian opposition and the need for humanitarian aid in the country.
The opposition meetings in Istanbul come two days after a top military defector said armed rebel groups have aligned under the leadership of the Free Syrian Army.
The United Nations estimates the Syrian conflict has killed more than 8,000 people; opposition activists put the toll at more than 10,000.
CNN's Ivan Watson, Arwa Damon, Saad Abedine, Holly Yan and Salma Abdelaziz contributed to this report.