"We will evacuate as many people as possible but there are those who will stay behind for reasons that don't depend on us but on the situation there," Robles told Cadena Ser radio.
"Even for those who reach Kabul, access to the airport is very complicated," she said, describing the situation as "dramatic" as aggression from the Taliban increases.
Her remarks came as US President Joe Biden said Washington would stick to its August 31 deadline to pull American forces out of Afghanistan following talks with his G7 counterparts, US media reported.
With the deadline looming, Robles said Spanish troops were trying to help Afghans stuck outside Kabul airport.
"Until this morning, it was impossible to leave Kabul airport. In the last few hours, an option has been under way... because the Taliban violence is very intense," she told reporters, without giving further details.
- More evacuees due Wednesday -
Spain has been evacuating its nationals and local contractors from Afghanistan via Dubai since the Taliban swept to power 10 days ago.
The Spanish government has not said how many people it plans on taking out but has insisted it would continue evacuating "as many people as possible".
"We will continue to bring out all people at risk until the last possible moment," Interior Minister Fernando Grande-Marlaska told reporters at Torrejon de Ardoz airbase after a plane landed carrying 290 refugees, mostly Afghan women and children.
Including the latest arrivals, Marlaska said "1,105 people" had landed in Spain. Of those, 613 had already made their asylum requests, with the rest either in transit or linked to a group that had worked with the US.
Two more planes are due on Wednesday, he said, with the first due at dawn.
Robles said on Monday that Spain could only carry out evacuation flights as long as Kabul airport was controlled by US troops.
Spain has agreed to host up to 4,000 Afghans who will be airlifted by the United States to airbases in Rota and Moron de la Frontera in southern Spain.
Under an agreement signed by Madrid and Washington, the evacuees may stay at the airbases, which are used jointly by the United States and Spain, for up to 15 days.