Syrian pro-government forces were poised to launch an assault to retake the Islamic State-held city of Palmyra, as Washington and Moscow meet Thursday to discuss the fraught peace talks.
The Islamic State (IS) group overran the city dubbed the "Pearl of the Desert" last May, and it has since blown up UNESCO-listed temples and looted relics dating back thousands of years.
Its recapture would be a strategic as well as symbolic victory for President Bashar al-Assad, since whoever controls it also controls the vast desert extending from central Syria to the Iraqi border, experts say.
Loyalists backed by Russian air strikes were "800 metres (yards) from Palmyra" and now control areas linking it to Damascus and Syria's third city Homs, a Syrian security source said.
"The army is now at (the southern and southwestern) entrances to the city and is preparing to begin the battle to liberate Palmyra," the source said.
Rami Abdel Rahman of the Syrian Observatory for Human Rights said earlier Wednesday that regime forces were two kilometres (one mile) south of Palmyra and five kilometres (three miles) southwest of the city.
Meanwhile, there is some hope that high-level US-Russian meetings on Thursday could deliver the momentum needed to move the Geneva peace talks to a new round.
US Secretary of State John Kerry will meet President Vladimir Putin and Foreign Minister Sergei Lavrov to gauge whether Moscow is ready to discuss ways to ease its ally Assad from power.
With the indirect negotiations in Geneva proving to be sluggish, all eyes are on Moscow since the two powers hold significant sway over the opposing sides in Syria's devastating conflict.
"What we're looking for, and what we've been looking for for a long time is how are we going to transition away from Assad's leadership," a senior US official told reporters.
"On the Russian side, there's only one decision maker and you need to be in the room with him to evaluate what's possible," the State Department official added.
The official said a shaky ceasefire between the regime and rebel forces and Russia's partial withdrawal could mark an opening for Putin to shift his stance on Assad.
The High Negotiations Committee, the main Syrian opposition body, said it hoped that after the Kerry-Kremlin talks "a clear message will be sent to Bashar al-Assad: He cannot continue to paralyse the political transition that the Syrian people are demanding".
"Syria's future must be decided by the Syrian people, not by a single man," the group said.
Assad's future has been a key obstacle in the latest talks, with the government stubbornly insisting any discussion of him leaving is "excluded" and the opposition saying any talk of allowing him to stay is "absolutely unacceptable".
UN envoy Staffan de Mistura voiced "a strong expectation that the talks in Moscow will be productive."
When the two top diplomats reach a "common understanding", the process is helped "enormously", the envoy said.
But in an interview with AFP, Damascus's lead negotiator at the Geneva talks, Bashar al-Jaafari, insisted that thinking that the regime could be pressured by its Russian ally was a "misreading" of the situation.
"When we say that the dialogue must be between Syrians, without outside intervention, this also applies to the Russians and Americans," he said.
Washington and Moscow were instrumental in bringing about a fragile truce declared on February 27, raising hopes for an end to the five-year Syrian conflict that has killed more than 270,000 people and forced millions to flee their homes.
The ceasefire does not include areas held by IS or the Al-Qaeda-linked Al-Nusra Front, allowing the regime to launch an offensive at the start of the month, backed by heavy Russian air strikes, to try to retake Palmyra.
Moscow, which made a surprise announcement last week that it was withdrawing most of its troops from Syria, has continued support for the government's bid to liberate what Putin described as a "pearl of world civilisation".
Russia's military intervention has already allowed the regime to retake significant territory since last September.
Analysts say Moscow's partial withdrawal could help the peace drive by weakening Assad's position.
"If the Russians drop Bashar, he will collapse," said Middle East consultant Agnes Levallois.
But the commander of Russia's forces in Syria admitted in an interview that Russia's special forces are still active on the battlefield, directing Russian warplanes.
"I won't conceal that there are also contingents of our special operations forces in Syria," Alexander Dvornikov told Rossiyskaya Gazeta.