SOFIA (Reuters) - Bulgaria is sending more soldiers to strengthen controls along its border with Turkey and avoid a refugee influx that has overwhelmed its neighbors, Defence Minister Nikolay Nenchev said on Wednesday.

"There is a change in the situation in the past few days and it is hard to predict where the refugee wave will head...so we are standing ready," Nenchev told public BNR radio.

Fifty soldiers have been sent to the border and a further 160 could be deployed by the end of Thursday. The Bulgarian army could send up to 1,000 troops to back up border police if needed, he added.

Bulgaria took the measures after reports that hundreds of mostly Syrian refugees have spent the night in the open near the Turkish border with Greece, which is also very close to Bulgarian-Turkish border. [ID:nL5N11M2LV]

Bulgaria is a member of the European Union but not the border-free Schengen Area.

About 660 migrants have tried to cross the Bulgarian-Turkish border in the past 25 hours but have returned voluntarily after they had seen that the border was well-guarded, the chief secretary of the interior ministry Georgi Kostov, told reporters.

Bulgaria is a member of the European Union but not the border-free Schengen Area.

(Reporting By Tsvetelia Tsolova; Editing by Angus MacSwan)