A member of Special Monitoring Mission of the Organization for Security and Cooperation (OSCE) to Ukraine walks along a convoy of Ukrainian armed forces in Blagodatne, eastern Ukraine, February 27, 2015. REUTERS/Gleb Garanich

VIENNA (Reuters) - The Organisation for Security and Cooperation in Europe (OSCE) will extend its observer mission to Ukraine for one year, until March 2016, and may double its size to 1,000 observers, a spokeswoman said on Thursday.

Ukraine’s military said on Tuesday pro-Russian rebels were amassing heavy weapons in depots around separatist-held Donetsk city despite last month’s ceasefire deal, which calls for the withdrawal of heavy weapons from the front line.

The OSCE said then that it could not confirm the withdrawal of weapons by either side, because it had not been given access to all locations where weapons might be stored.

The OSCE monitoring mission to Ukraine was first deployed a year ago following a request from Ukraine. It delivers public reports on fighting between pro-Russian groups and Ukrainians loyal to Kiev as well as on movements on border crossings between Russia and Ukraine.

The OSCE currently has around 470 civilian monitors on the ground in Ukraine, the spokeswoman said.