A German aid worker held for almost four months has escaped his kidnappers in Syria, just like his two colleagues who managed to flee in July, their aid group announced on Wednesday.

Engineer Ziad Nouri "is free" and managed to cross the Syrian border into Turkey on Tuesday, the Gruenhelme non-governmental organisation said in a statement.

"He freed himself," it said.

He was kidnapped with two other members of the group on May 15 in the Syrian village of Harem, near the Turkish border where the organisation was renovating a nursery.

His two colleagues Simon Sauer and Bernd Blechschmidt escaped on July 3, the NGO said. The German foreign ministry confirmed their release at the time.

Gruenhelme (Green Helmets) has been in northern Syria for several months to help the reconstruction of local infrastructures and to provide healthcare.