Saudi Arabia vowed on Tuesday to respond "firmly" to attacks by Yemen's Houthis, after the Iran-backed fighters wounded 13 people in the kingdom in an assault using ballistic missiles and drones.

The Houthis have been engaged in renewed fighting with Saudi-backed government forces since Yemen's civil war reignited in July, when the group declared a maritime blockade on Riyadh and began targeting its ships in the Red Sea.

Qatar on Tuesday warned of dire consequences if the vital Bab al-Mandab waterway were to close, after a months-long blockade of the Strait of Hormuz and recent Houthi gains on the Red Sea coast.

"The world is suffering enough with the closure of the Strait of Hormuz, so we cannot also afford to add the Bab al-Mandab into that equation," Qatar foreign ministry official Ibrahim Al Hashmi told reporters.

The United Nations called Tuesday for international investigators to be granted to access to all of Gaza "to assist with evidence-gathering" as hundreds of bodies are pulled from the rubble.

"The discovery of extensive remains under the rubble in Gaza City resurfaces these concerns of war crimes and other atrocity crimes," U.N. rights chief Volker Turk said in a statement.

Palestinian activist Mahmoud Khalil sued Columbia University on Monday, alleging it failed to address harassment against him and other members of a pro-Palestinian campus group and retaliated against them when they reported it.

The university's "deliberate indifference" led to the doxing of multiple group members and Khalil's detention by immigration agents, according to the civil rights lawsuit filed in federal court in New York City.

Two deadlines are looming for Iraq at the end of the month: the end of a decades-long U.S. military presence and the disarmament of non-state armed groups.

The U.S. withdrawal is underway, and officials say the hundreds of troops remaining in northern Iraq will be out by the Sept. 30 deadline.

Yemen's Iran-backed Houthi rebels captured more strategic islands in the southern Red Sea, government and Houthi officials said Monday, strengthening their control of a major maritime shipping route.

The capture of the islands of Greater and Lesser Hanish is the latest in the rebels' swift advance around the Bab el-Mandeb Strait, a key passage for Saudi oil shipments that has only become more crucial during the Iran war since it serves as an alternative to the Strait of Hormuz.

Yemen's Houthis said Monday they had fired dozens of drones and ballistic missiles at military targets in Saudi Arabia, in retaliation for airstrikes by Riyadh.

"The Yemeni armed forces conducted a large-scale military operation targeting military installations and infrastructure, including fighter jet hangars, radars, runways, ammunition depots, and other targets located at King Khalid Air Base in Khamis Mushait," said Houthi military spokesman Yahya Saree.

Iran's foreign ministry on Monday denied the country had any involvement in the war in Yemen, after the Iran-backed Houthi movement seized more territory.

"Iran does not interfere in Yemeni affairs," ministry spokesman Esmaeil Baqaei told reporters, saying the Houthis are "entirely independent actors who make their decisions based on their own interests".

Iran said a planned meeting with Gulf countries in Oman on Monday to discuss the Strait of Hormuz was postponed at the request of Saudi Arabia, which cited the war in Yemen.

"It is true that Saudi Arabia asked for this meeting to be postponed," said foreign ministry spokesman Esmaeil Baqaei. "Invoking the Yemeni issue as the reason is a false pretext," he added at a press conference in Tehran attended by AFP.

President Donald Trump said Sunday that he will look into requests from families of Sept. 11 victims to release records that could shed light on Saudi Arabia's alleged role in the attacks 25 years ago.

Several families have long alleged that a group of extremist religious leaders in Saudi Arabia gained influence in the Saudi government and aided the hijackers. Fifteen of the 19 hijackers were Saudis, but the Saudi government has long denied any involvement in the attacks.