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The United Nations' Syria envoy sent an urgent appeal Saturday to Russia, Iran and Turkey seeking help to restore a ceasefire, warning that escalating violence was threatening peace talks in Geneva.

UN mediator Staffan de Mistura sent letters to Moscow and Tehran - which back the Damascus regime - as well as Ankara which supports the Syrian opposition, expressing concern over surging violence around the Syrian capital and central Hama province.

"Growing violations in recent days are undermining the ceasefire", agreed at separate negotiations in Kazakhstan's capital Astana overseen by the three countries, de Mistura's office said in a statement.

The violence has had "significant negative consequences for the safety of Syrian civilians, humanitarian access and the momentum of the political process" in Geneva, it added.

De Mistura asked the countries concerned "to undertake urgent efforts to uphold the ceasefire", said the statement issued during a fifth round of UN-brokered talks in the Swiss city.

Opposition and jihadist fighters have in the past week launched attacks on government positions around Damascus and Hama. The regime has responded with air strikes and other bombardments.

Syrian government negotiators met the UN mediator earlier Saturday for a session focused on terrorism, which is one of four issues on the agenda this round.

The others are governance, elections and drafting a new constitution.

Killing machine

The main opposition High Negotiations Committee (HNC) met later with de Mistura's team, and the talks focused on forming a new government to replace President Bashar al-Assad's regime - the rebel's top priority.

Citing the recent violence, HNC delegation chief Nasr al-Hariri called the Assad government "a killing machine", while characterising recent rebel attacks on regime positions as "a must for us to defend ourselves."

Asked if rising violence could see the talks collapse, Hariri said: "if we don't have that real ceasefire then things will deteriorate very badly."

Hariri also fiercely condemned Saturday's airstrike on the opposition held town of Hammuriyeh that killed at least 16 civilians, according to the Syrian Observatory for Human Rights.

The latest round of talks wrapped up a second full day with little hope of a breakthrough and rivals still deadlocked on key issues. They have not yet met face-to-face.

Years of diplomatic efforts have failed to end the Syrian conflict, which has killed more than 320,000 people and displaced millions since it started in March 2011 with protests against Assad's regime.