Israeli troops seized control of Gaza’s vital Rafah border crossing on Tuesday in what the White House described as a limited operation, as fears mount of a full-scale invasion of the southern city and talks with Hamas over a cease-fire and hostage release remain on a knife’s edge.

The U.N. warned of a potential collapse of the flow of aid to Palestinians from the closure the Rafah crossing from Egypt and the other main crossing into Gaza, Kerem Shalom, from Israel, at a time when U.N. officials say northern Gaza is experiencing “full-blown famine.”

The Rafah and Kerem Shalom crossings are critical entry points for food, medicine and other supplies for Gaza’s 2.3 million people. They have been closed for at least the past two days, though the smaller Erez crossing between Israel and northern Gaza continues to operate.