Israel sent tanks into the southern Gaza city of Rafah and seized control of the crossing, a key aid passage, while warning it will deepen its operation if truce talks fail to secure the release of hostages held by Hamas.
President Joe Biden declined to answer questions on the ceasefire talks and the operation in Rafah as he met his Romanian counterpart in the Oval Office, merely smiling at reporters.
"The crossings that have been closed need to be reopened, it is unacceptable for them to be closed," White House Press Secretary Karine Jean-Pierre told a briefing.
"We believe Rafah border crossing should be quickly reopened for the movement of humanitarian assistance," Jean-Pierre added.
Israel had already committed to reopening another crossing at Kerem Shalom, which was closed after a rocket attack on Sunday killed four Israeli troops, on Wednesday, she added.
The United States, Israel's main military and diplomatic backer, has repeatedly said it opposes a major offensive in Rafah, where more than 1.2 million displaced Palestinians are living.
But the White House said Israel had told it that the current operation was of "limited scope, scale and duration."
"What we've been told by our Israeli counterparts is that this operation last night was limited and designed to cut off Hamas's ability to smuggle weapons" into Gaza," National Security Council spokesman John Kirby said.
Washington would closely monitor the situation, he added.
'Remaining gaps'
The Israeli military's thrust into the eastern sector of the city came shortly after the Palestinian militant group Hamas said it had accepted a ceasefire proposal, but Israel demurred.
The talks got back underway on Tuesday amid high hopes and tensions after more than seven months of conflict.
"A close assessment of the two sides' positions suggests that they should be able to close the remaining gaps, and we're going to do everything we can to support that process," Kirby said.
The fact that all parties, including CIA chief Bill Burns, were present at the talks in Cairo indicated they were at an advanced stage, he added.
"Everybody's coming to the table," Kirby said. "That's not insignificant."
The White House hoped for news "very, very soon" but it would be "foolhardy" to predict when the negotiations might bear fruit, he added.
The war was sparked by Hamas's October 7 attack on Israel, which resulted in the deaths of more than 1,170 people, mostly civilians, according to an AFP tally of Israeli official figures.
Israel launched a retaliatory offensive that has killed at least 34,789 people in Gaza, mostly women and children, according to the Hamas-run territory's health ministry.
