Summary:

US officials report multiple US, civilian casualties

Suicide bomb explodes at Kabul airport gate, second explosion reported as car bomb

Taliban warned of potential terrorist threat

US State Dept warned Americans to "leave immediately" due to imminent threat

At least 3 US Marines injured

At least 12 civilians dead including kids

Gunfire reported

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Update (1102ET):

According to Pentagon Press Secretary John Kirby, there have been multiple US and civilian casualties. Kirby also confirmed the second explosion:

"We can confirm that the explosion at the Abbey Gate was the result of a complex attack that resulted in a number of US & civilian casualties. We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate. We will continue to update."