Israel has been regularly supplying Syrian rebels near its border with cash as well as food, fuel and medical supplies for years, a secret engagement in the enemy country’s civil war aimed at carving out a buffer zone populated by friendly forces.

The Israeli army is in regular communication with rebel groups and its assistance includes undisclosed payments to commanders that help pay salaries of fighters and buy ammunition and weapons, according to interviews with about half a dozen Syrian fighters. Israel has established a military unit that oversees the support in Syria—a country that it has been in a state of war with for decades—and set aside a specific budget for the aid, said one person familiar with the Israeli operation.

Israel has in the past acknowledged treating some 3,000 wounded Syrians, many of them fighters, in its hospitals since 2013 as well as providing humanitarian aid such as food and clothing to civilians near the border during winter. But interviews with half a dozen rebels and three people familiar with Israel’s thinking reveal that the country’s involvement is much deeper and more coordinated than previously known and entails direct funding of opposition fighters near its border for years.

“Israel stood by our side in a heroic way,” said Moatasem al-Golani, spokesman for the rebel group Fursan al-Joulan, or Knights of the Golan. “We wouldn’t have survived without Israel’s assistance.”

Israel’s aim is to keep Iran-backed fighters allied to the Syrian regime, such as the Lebanese militant group Hezbollah, away from the 45-mile stretch of border on the divided Golan Heights, the three people said.