Iran launched more than 100 drones at Israel Saturday, in a retaliatory attack weeks after a strike hit an Iranian government building in Syria. Iran holds Israel responsible for the attack that killed two top Iranian commanders.

The counter strike heightens concerns about “a major escalation between the regional archenemies,” Reuters reported.

How the Biden administration responded to Iran’s attacks

According to a White House press release, President Joe Biden’s administration expects the attack to “unfold over a number of hours,” and confirmed the U.S.’s alliance with Israel.

This attack is likely to unfold over a number of hours. “President Biden has been clear: our support for Israel’s security is ironclad. The United States will stand with the people of Israel and support their defense against these threats from Iran.”

What we know about the Iranian strikes headed for Israel

The U.S. military shot down some of the Iranian drones en route to Israel Saturday afternoon.

Israel has a “very strong defensive posture in order to address anything flying toward Israel,” an Israel Defense Forces official said, per The Washington Post.

Iran is targeting the Negev Desert region, an area that houses several military bases, per The New York Times.

“The attack, while expected, threatened to open a volatile new chapter in the long-running shadow war between Iran and Israel, marking a significant escalation in hostilities between the two regional foes,” the Times reported.