U.S. Secretary of State John Kerry said on Monday that while efforts were being made to track down alleged violations of the cessation of hostilities in Syria, there was currently no evidence to suggest it would destabilize the fragile peace.

“We are digging in through the process we set up to find out if in fact a violation did take place or was it in fact a legitimate engagement against Nusra only or Daesh only,” Kerry said in a news conference with German Foreign Minister Frank-Walter Steinmeier.

The Nusra front is an al Qaeda-linked group while Daesh is an Arabic acronym for Islamic State (ISIS).

Kerry said that while there were reports of violations, the vast majority in Syria had seen a decrease in violence.

“So we call on all the parties not to be looking for a way to get out from under the responsibility of the cessation of hostilities, but rather to help the process to hold itself accountable,” Kerry added.

On Monday a senior official from the Saudi-backed opposition High Negotiations Committee (HNC) said the cessation of hostilities faced “complete nullification” because Syrian government attacks were violating the agreement.

The cessation of hostilities agreement, the first of its kind since the Syrian civil war began in 2011, is a less formal arrangement than a ceasefire. It is meant to allow peace talks to resume and aid to reach besieged communities.

The cessation deal does not include jihadist groups such as ISIS and the Nusra Front. Russia, which is backing the Syrian government with air power, has made clear it intends to keep bombing these groups.

Kerry said he agreed with his Russian counterpart to intensify work on a mechanism to ensure any strikes in Syria solely target ISIS or Nusra Front.

He added that he was concerned by reports that the Syrian government was creating obstacles for the delivery of humanitarian aid and hoped it would stop its officials and troops from taking medicines or other supplies from the shipments.