Top U.S. officials are scrambling to assist thousands of Afghan allies and translators with evacuations, as President Biden pledged U.S. support Thursday.

"Those who helped us are not going to be left behind," Biden told reporters.

The Biden administration began the official removal of U.S. troops from Afghanistan early last month, postponing the original exodus date of May 1 – agreed to under the Trump administration – to Sept. 11, 2021.

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A contingent of 650 troops are expected to remain behind to serve as security for U.S. diplomats after the withdrawal is complete, a U.S. defense official told Fox News Thursday.

Pentagon spokesman John Kirby said the details surrounding the evacuations and relocations are not yet set in stone. 

"We just don’t know, it will be done on a case by case basis," he said in answer to questions on where the Afghan allies will be sent. 

Kirby said the State Department is taking the lead on safely relocating translators and Afghanis that assisted the U.S. during the nation’s 20-year long war in the Middle East.

"It’s going to take a whole inter-agency effort to do this," the Pentagon spokesman said, adding the Department of Defense and Homeland Security will play a role in assisting mass evacuations. 

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"This is not an uncommon mission set for the U.S. government and it doesn’t have to entail military aircraft to accommodate," he continued, noting that the U.S. has used commercial aircraft during prior evacuations. 

Pentagon officials told Fox News the complete withdrawal of U.S. forces in Afghanistan is just weeks away – cumulating with the Taliban’s increased strength.

United Nations envoy for Afghanistan Deborah Lyons warned this week that the terrorist organization has taken 50 of the country’s 370 districts in a recent surge of violence. 

And top intelligence officials have warned Kabul could fall to the Taliban within six months of the withdrawal of U.S. and NATO troops.

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"We’re mindful that the security situation in some areas of Afghanistan is certainly deteriorating," Kirby said Thursday. 

"It remains to be seen how this is going to play out," the Pentagon spokesman added. "This is their country…and ultimately it’s their responsibility to protect their citizens and their sovereignty."

Lucas Tomlinson contributed to this report.