United Nations war crimes investigators called on world powers on Tuesday to pressure the warring sides in Syria to return to the negotiating table to hold the conflict and civilian suffering.

Paulo Pinheiro, chair of the UN independent commission of inquiry on Syria, said that the Syrian government was conducting daily airstrikes, while militant groups including Islamic State also carried out indiscriminate attacks.

"We need all States to insist time and time again that influential States and the (UN) Security Council unconditionally support the political process," Pinheiro told the UN Human Rights Council in Geneva.