The tense calm between Israel and Syria has been made possible, in part, by U.S. reconnaissance flights over their border for the last 18 years.

This revelation was made in a book published Thursday by the congressionally funded U.S. Institute for Peace. The existence of the flights was confirmed by diplomats, who declined to discuss the operation further.The United States provides information it collects on the flights to both Syria and Israel, ensuring its acceptance as an evenhanded broker, wrote Michael Krepon, one of the book's contributors and the director of the Henry L. Stimson Center for arms control.

The overflights are conducted mainly over the Golan Heights, a strategically located plateau that Israel captured from Syria in the 1967 Middle East war, Krepon said in the book, "Arms Control and Confidence Building in the Middle East."

The flights, which have rarely if ever been acknowledged publicly, have been conducted since 1974 with the tacit agreement of both countries, he wrote.

The flights are conducted under the auspices of the United Nations, which monitors the 1974 disengagement agreement between the two archenemies, Krepon said.

He did not provide details, arguing that the secrecy surrounding the U.S. overflights was one of the clues to their success.

Similar aerial surveillance and photography is carried out over the Sinai Peninsula to monitor the 1979 peace agreement between Israel and Egypt.

In addition to the U.S. monitoring over the Golan, Syria and Israel conduct their own aerial surveillance near the border, Krepon said.