UN peacekeepers were forced to temporarily suspend their patrols along the Israel-Lebanon Blue Line on Sunday in response to the Israeli military releasing what they referred to as a “chemical substance.”

The United Nations Interim Force in Lebanon (UNIFIL) said that they were told by the Israeli Defense Forces (IDF)  that the IDF would be carrying out an “aerial activity dropping what they said was a non-toxic chemical substance over areas near the Blue Line.” The IDF instructed peacekeepers to withdraw from areas where they carry out mandated operations. As a result, on Monday, UN personnel pulled over a dozen peacekeepers from the frontier for over nine hours.

The Blue Line is a 120-kilometer “line of withdrawal” stretching along the border between Israel and Lebanon. Set by the UN in 2000, it confirms the withdrawal of Israeli forces from the south of Lebanon. It is intended to be temporary, and the peacekeepers are its “temporary custodians.” Due to its nature, the UN has reiterated concerns about any flight movements across the boundary.

UNIFIL raises wider concerns regarding the potential impact of the unknown substance on citizens and the environment. UNIFIL supported the Lebanese army in collecting samples of the affected areas for testing. As of Monday afternoon, the UN had not received test results.

This is not the first time the IDF has dropped unknown chemical substances. Such an activity violates UN Security Council Resolution 1701, adopted in 2006, with the potential of putting both the health of the peacekeepers and the civilians at risk. The UN again called on the IDF to cease such activities and work alongside the peacekeepers.