Leaders of 27 European Union countries are meeting without Britain in Bratislava next week to discuss plans to boost the bloc's security and defence measures in the wake of the Brexit vote.
During a visit to Estonia on Friday, Tusk is expected to say that he will "appeal to all 27 leaders next week in Bratislava to send concrete, operational support to Bulgaria to protect its border with Turkey, in the form of border guards and equipment," a European source told AFP on condition of anonymity.
But the EU's efforts to tackle the crisis have been hampered by a lack of solidarity for frontline states that face the biggest numbers of migrant arrivals, especially Greece and Italy.
Tusk has been touring European capitals in recent days to build a consensus on how to move on after Brexit, particularly in terms of border security and in boosting cooperation on defence.
He is expected to reveal further details on the plans for the September 16 Bratislava summit in Stockholm later today.