Iran live updates: At least 20 killed in week of attacks on Saudi airports, officials say

At least 20 people were killed in a series of attacks by Yemen's Iran-backed Houthi rebels on airports in Saudi Arabia, the country's General Authority of Civil Aviation said.

Twenty people were killed and 356 were injured in attacks between Oct. 6 and Oct. 11, according to GACA. The attack on Oct. 10 at King Khalid Airport was among the deadliest in the kingdom in more than a decade, Reuters reported.

President Donald Trump told ABC News the U.S. "may" join Saudi strikes targeting the Houthis following the attacks.

Saudi aviation officials said they were tightening security following the attacks, which intensified over the weekend. The escalating threat from the Houthis prompted the closure of King Khalid International Airport in Riyadh and other airports as of Monday morning, officials said.

Schools across Saudi Arabia are also shifting to online classes for at least this week, according to sources in Saudi Arabia, including a diplomat who spoke with ABC News.

Two alerts were heard in the Saudi capital on Monday morning, the diplomat said.

-ABC News' Camilla Alcini, Nasser Atta and Zoe Magee

The Federal Aviation Administration has issued a warning to U.S. airlines flying to or over Saudi Arabia, saying airlines should exercise caution due to “safety-of-flight risks associated with ongoing military activities and GNSS interference.”

“Threats could pose a potential risk to aircraft at all altitudes, including during overflight, the arrival, and departure phases of flight,” the FAA notice to airmen (NOTAM) said.

The NOTAM is in place from now until Dec. 10.

FedEx flies to Riyadh, but no U.S. commercial airlines currently fly there. Delta was scheduled to launch an Atlanta-to-Riyadh route on Oct. 23, but Delta CEO Ed Bastian told CNBC that “safety is going to make the call” on whether they actually launch or not.

-ABC News' Clara McMichael

The Saudi General Authority of Civil Aviation said the attack on King Khalid International Airport in Riyadh on Sunday killed one Pakistani national and injured 11 others.

The authority said the injured were "immediately transferred to hospital to receive medical care."

The attacks also resulted in material damage, which is currently being assessed, the authority added.

-ABC News' Jamie Dorrington

The King Khalid International Airport in Riyadh, Saudi Arabia, was subject to a new attack on Sunday afternoon, according to the Saudi General Authority of Civil Aviation.

Officials said the attack resulted in injuries and assessments of the damage were ongoing in coordination with relevant authorities.

Maj. Gen. Turki Al-Maliki, spokesperson for the Saudi-led coalition in Yemen, said on social media that an "unknown projectile" had struck Passenger Hall No. 4 at King Khalid International Airport.

"A fire resulting from the attack is being directly handled by the competent authorities at the site," Al-Maliki said.

In a later post, Al-Maliki said Saudi air defenses had "destroyed a loitering (cruise) missile in the vicinity of King Khalid International Airport."

The new incident comes a day after a dozen people were killed and at least 300 were injured in an attack on King Khalid International Airport, the Saudi General Authority of Civil Aviation said. The Iran-backed Houthi rebels claimed responsibility for Saturday's attack. Secretary of State Marco Rubio confirmed that an American citizen was killed in Saturday's strike on the airport.

-ABC News' Jamie Dorrington