"De Mistura has been vested with a clear mission based on a solution to the crisis through negotiations between Syrians," Al-Watan wrote.
"But he has gone a bit astray in statements perhaps made under international pressure," the daily said.
The criticism came after De Mistura told reporters this week that he had an "action plan" to ease suffering in Syria that includes "freeze zones," where fighting would be suspended to allow humanitarian relief.
Al-Watan responded by saying the envoy "should learn more about the reconciliation" efforts being made between the Syrian government and opposition forces over limited areas.
Such efforts "mean that the combatants surrender their weapons and return to the fold of the state," the newspaper said.
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The Syrian government has pursued a number of ceasefires in rebel-held areas, usually winning agreement after lengthy sieges and heavy bombardment.
Its opponents criticise the tactic as less a method of reconciliation and more a way of starving and bombing rebel neighbourhoods into submission.
But Damascus has held them up as a way to restore security to neighbourhoods wracked by conflict.
The government opposes the creation of any kind of buffer zone or the imposition of ceasefires, seeing them as a violation of its sovereignty and potentially providing rebels a chance to regroup.
Al-Watan warned that De Mistura "should not be taken in by... the dreams of regional and Western countries who want to divide Syria under humanitarian pretexts".
De Mistura took on the role of UN envoy this summer, replacing Lakhdar Brahimi.
He has already visited Damascus, as well as Iran and Russia, two key allies of the Syrian regime.