Syria's foreign minister on Saturday linked any talks on the future of a rebel-held southern region with an end to US presence in another area bordering Iraq and Jordan.
Regime ally Russia has called for a meeting with the United States and Jordan on the future of the southern provinces of Daraa and Quneitra, bordering Jordan and the Israeli-occupied Golan Heights.
In recent weeks, Damascus has sent military reinforcements to the two provinces, which comprise some of the closest rebel-held areas to the capital.
President Vladimir Putin has spoken with Israeli Prime Minister Benyamin Netanyahu about proposed talks.
"We have not yet entered into negotiations over the southern front," Syria's Foreign Minister Walid Muallem said at a press conference in Damascus.
"The indicator will be the withdrawal of the United States from our land in al-Tanaf" near the Iraqi and Jordanian borders, Muallem said.
The United States and its allies have used a base in the area to train a force fighting the Islamic State (IS) militant group.
"Don't believe anything that is said about an agreement on the south until you see that the United States has withdrawn its forces from the At-Tanaf base," he said.
"It must withdraw its forces from At-Tanaf."
"We have strived from the start to resolve the issue in the ways that we are used to, which are reconciliations," he said. "If it is not feasible, we will see what will happen."
Moscow-brokered reconciliation deals have seen rebels withdraw from several areas of Syria including opposition strongholds close to the capital, often after blistering regime offensives and sieges.
Last month, Washington warned Damascus it would take "firm" action if the regime violated a ceasefire deal for southern Syria that was negotiated with Russia and Jordan last year.
The warning came after regime aircraft dropped leaflets on Daraa, urging the rebels who control most of the province to lay down their weapons or face an offensive.