The United Nations (UN) is investigating allegations Turkey has used chemical weapons against civilians in Syria.
A number of children have reportedly been treated for severe burns possibly from white phosphorus in the border town of Ras al-Ayn.
Turkey denies the claims.
President Donald Trump sent his deputy Mike Pence and US Secretary of State Mike Pompeo to Turkey to push for a ceasefire, on Thursday.
A 120-hour pause in hostilities was agreed to allow Kurdish forces to withdraw from a 20-mile deep safe zone Turkey is demanding along the border.
Mr Pompeo said: "We're hopeful in the hours ahead that both Turks who were part of the agreement alongside us as well as the YPG fighters in the region will take seriously the commitments they made."
Mr Stoltenberg said the agreement between Turkey and the US was "important" as it made a path for de-escalation in the region.
"It is important that two NATO allies, the United States and Turkey who have been on the ground for a long time, have made an agreement on the way forward," he said.
"It can provide a basis for progress and for de-escalating the situation."
Mr Trump's withdrawal of US troops from Syria was blamed by critics for the increasing violence and humanitarian crisis.
The Syrian President Bashar al-Assad, backed by Russia, deployed forces to fill the US vacuum.
President Erdogan is to meet Russian President Putin next week to discuss further steps regarding the safe zone.