KIEV (Reuters) - International monitors in eastern Ukraine have reported a sharp increase in gunfire along front lines near the airport of the rebel-held city of Donetsk, a senior official with Europe’s main security and rights watchdog said on Thursday.

Servicemen place ammunition during military exercises conducted by Ukrainian armed forces at the Shiroky Lan training ground in Mykolaiv region, Ukraine, October 30, 2015. REUTERS/Oleksandr Klymenko

The guns have been mostly silent since early September, prompting both sides to extend a pullback of lighter weapons, but recent accusations of ceasefire violations from both sides have underscored the fragility of the peace process.

Renewed violence would also make it less likely that the European Union lifts sanctions against Russia based on a review before the end of the year, though Russia has repeatedly denied it is backing the rebels in east Ukraine.

The Organization for Security and Cooperation in Europe (OSCE), which monitors the implementation of the ceasefire agreement, said it had noted violations near Donetsk airport, a long-time hotspot for fighting.

“Since Sunday, monitors noted a noticeable increase in violence in and around Donetsk airport, none of which we can attribute to demining or training,” Alexander Hug, deputy chief monitor to the OSCE mission in Ukraine, was quoted by the OSCE on Twitter as saying during a weekly briefing.

The nature of the violence was not spelled out. However, the OSCE monitors’ report on Wednesday cited over 150 explosions and multiple bursts of small-arms and anti-aircraft machine-gun fire heard in various areas of the front.

The OSCE’s Hug said the agreed deadline for withdrawing lighter weapons, including tanks and armored personnel carriers, from the front line was in the next few days, but the pullback had not been entirely successful so far.

“We’ve seen many discrepancies in the withdrawal process in east Ukraine and I urge sides to use the last days ahead of the deadline to fully comply,” he said in the tweeted report.

“Of particular worry is that at least one Grad rocket was used in Staromykhailivka ... on Nov. 2,” he said, referring to a western district on front lines west of Donetsk.

On Wednesday a Ukrainian security chief said the withdrawal process would stop if truce violations by pro-Russian separatists continue, while separatists said attacks from the Ukrainian side suggested the peace deal existed only on paper.

U.S. officials later expressed concern over the recent uptick in violence.

The ceasefire deal aims to end a conflict in which over 8,000 have been killed since fighting between Ukrainian troops and rebels seeking independence from Kiev erupted in April 2014.