Update: It has since been reported that four US Marines were killed.

An explosion went off at the Kabul airport in Afghanistan Thursday morning. This after US state department warnings to Americans not to travel to the Kabul airport, and to leave if they were stranded outside the gates.

Reports from Sky News are that more than 30 people were wounded in the attack, and at least 13 have died.

A second explosion was reported shortly after the first.

Thousands have been waiting outside the airport in hopes to get on flights out of the Taliban-controlled nation. The Pentagon did not immediately release numbers of casualties, the AP reports.

An attack on the airport had been warned as a possibility, and many nations had told their citizens and those Afghans who they were planning to evacuate to steer clear of the area in anticipation of violence.

While there's no word yet on who is responsible for the attack, there have been concerns that ISIS-K was operating in the area.

On Thursday, the US embassy in Afghanistan warned American citizens who were seeking to leave the country not to travel to the Kabul airport due to terrorist threats. They asked those already at specific airport gates to leave immediately, in a statement early Thursday. The state department does not want Americans stuck outside the gates of the airport amid security threats.

"We're operating in a hostile environment, in a city and country now controlled by the Taliban, with the very real threat of an ISIS-K attack. We're taking every precaution, but this is very high risk," Secretary of State Tony Blinken said. The New York Times reports that 1,500 Americans are left in Afghanistan.

This is a breaking story and will be updated.