"The delegation will not leave today or tomorrow for Geneva, and the final decision (on attending) has not been taken yet," a source close to the government delegation told AFP on Tuesday.
An eighth round of peace talks aimed at ending Syria's nearly seven-year war began in Geneva last week.
UN Syria envoy Staffan de Mistura announced shortly after the talks began that they would be extended by two weeks.
Negotiations were paused over the weekend, but both sides had been expected to return to Geneva to resume discussions on Tuesday.
But only the opposition delegation showed up.
The Al-Watan newspaper, which is close to the government, reported that the regime's delegation was in Damascus and was not expected to leave either Tuesday or Wednesday.
It said the invitation to return to the talks "is still being studied by the Syrian leadership".
When asked about the delay, UN spokeswoman Alessandra Vellucci said the government delegation had "been invited back in Geneva as of today (Tuesday). The Special Envoy stands ready to engage them when they return."
"We expect and we hope they will be here very soon," she added.
Following the opposition's meeting with de Mistura Tuesday afternoon, delegation chief Nasr al-Hariri said it was time the UN clearly stated which party was sabotaging the peace process.
"Now it is the responsibility of the international community, of the UN and the Special Envoy to announce to the world who is the party who is rejecting the negotiations," he told reporters.
'Big problems'
The government delegation left Geneva last Saturday, after its chief negotiator Bashar al-Jaafari said there were "big problems in this round of talks".
He was referring to a communique published by the opposition last month in Riyadh signalling that it was maintaining its insistence on President Bashar al-Assad's removal.
The opposition, united in one delegation for the first time, has defied calls to give up on its demand that Assad must step down before any peace deal can be reached.
Jaafari described that position as "provocative" and "irresponsible", warning that "there will be no progress" if the opposition maintained that position.
De Mistura has tried to maintain an upbeat note on the talks, and on Friday he published a document suggesting 12 principles for a future Syria that he suggested the two sides could agree, including that the country "shall be democratic and non-sectarian".
He asked the parties to discuss the points and add their thoughts before the talks resumed.
More than 340,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.
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