AKCAKALE, Turkey — The laborers work all day, piling bags of fertilizer onto carts and wheeling them through the crossing that connects this southern border town to Syria.

The Syrian town next door is firmly controlled by the extremists of the Islamic State, as is clear from the black flag flying over downtown. And while the fertilizer, ammonium nitrate, is widely used for agriculture, it has also been used by terrorists around the world — including the Islamic State — to build powerful explosives.

Few here think the fertilizer is meant to help Syrian farmers.

“It is not for farming. It is for bombs,” said Mehmet Ayhan, an opposition politician from Akcakale who is running for Parliament. But he did not oppose the deliveries, saying they created jobs in his impoverished town.

“As long as the Turkish people benefit from this — regardless of where it goes on the other side — it is a good thing,” Mr. Ayhan said.