Should Israel find itself low on ammunition, it can tap into a stockpile of a billion dollars worth of American weapons stored in Israel by the United States for emergencies. So far, Israel has not requested access to the stockpile during the current hostilities with Hamas in Gaza.

The little-known stockpile is officially known as War Reserve Stockpile Ammunition-Israel (WRSA-I) and has been maintained inside Israel since the 1990s by United States European Command. It is a congressionally approved program that has grown in scope in recent years.

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The location of the stockpile as well as the types and quantities of ammunition it stores are classified. However, a Congressional Research Service report from April says "the United States stores missiles, armored vehicles and artillery ammunition" in the stockpile.

A U.S. defense official says "this program consists of U.S. owned and U.S. managed ammunition stockpiles in Israel for use by either U.S. or Israeli forces." Though the weapons in the stockpile belong to the United States they are essentially for Israel's use when they ask for it should they run low on certain stocks of ammunition in emergency situations.

The official said Israel has not requested to use ammunition stored in this stockpile as it did during the 2006 war with Hezbollah.

The size and scope of the arsenal has grown since it was first established in the 1990s. It initially held $100 million worth of ammunition, but by 2010 Congress had authorized $800 million worth of ammunition to be stored there. The defense official says the amount of ammunition in the stockpile is now worth about $1 billion.

Israel's access to the American ammunition can be done fairly quickly once there is presidential approval.

"If the president authorizes release to Israel, an "emergency" Foreign Military Sales case is processed by the DoD [Department of Defense] and the ammunition is sold to Israel," said the official.

Use of the stockpile in case of an emergency "is defined and authorized by the president," said the official. The transfer of the munitions to Israeli control "can take place in a matter of hours of a presidential authorization."

The Congressional Research Service Report says "the government of Israel pays for approximately 90 percent of transportation, storage and maintenance costs associated with the WRSA-I program."

ABC News' Tom Giusto contributed to this report