"We need to make sure the cessation of hostilities is brought back on track," Staffan de Mistura said at the start of a meeting with Russian Foreign Minister Sergei Lavrov in Moscow on Tuesday.
The truce, brokered by Russia and the United States, went into effect late February in a bid to facilitate negotiations between warring sides to the conflict.
However, an escalation in fighting in recent weeks has left the ceasefire in tatters and torpedoed the peace talks.
The latest round of UN-brokered indirect talks, which began in Geneva, Switzerland, on April 13, were brought to a halt after the main foreign-backed opposition group, known as the High Negotiations Committee, walked out of the discussions to protest at what it called the Syrian government’s violation of the armistice.
Elsewhere in his remarks, the UN mediator on Syria praised the cessation of hostilities as a "remarkable achievement," adding all efforts must make the truce regime more effective.
De Mistura is also expected to push for the truce to include Aleppo, which has been a flash point over the past weeks.
Aleppo has been divided between the government forces and foreign-backed militants since 2012, a year after the conflict broke out in Syria. Dozens have been killed in renewed clashes over the past few days as warnings are high that a human tragedy may unfold in the city.
Lavrov, for his part, said Russian and US military officials are working on spreading the ceasefire to new districts across Syria.
"Work is being conducted, primarily by the Russian and American military..., to consolidate the cessation of hostilities regime, spread it to new districts and create optimal conditions for delivering humanitarian aid," the Russian foreign minister said.
The top Russian diplomat said Moscow and Washington will create a joint center in Geneva to monitor the situation in Syria.
He expressed concern over Turkey’s shelling of Syria, adding that closing border between the two states is becoming more urgent.
Germany to Host Peace Talks on Syria
In a relevant development on Tuesday, Berlin said in a statement that German Foreign Minister Frank-Walter Steinmeier will meet with French Foreign Minister Jean-Marc Ayrault, the UN envoy for Syria, and Syria's main opposition leader, Riyad Hijab, on Wednesday over the crisis in the Arab country.
The discussions will focus on "how the conditions for a continuation of the peace talks in Geneva can be met, as well as how a reduction of violence and an improvement in the humanitarian situation in Syria can be achieved,” the statement read.
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