Pentagon officials said the U.S. will have up to 4,000 troops on the ground in Kabul by the end of the day as the military races to continue its evacuation efforts ahead of the Aug. 31 withdrawal deadline the administration has set.

U.S. Army Major Gen. William Taylor of Joint Staff Operations told reporters in a press briefing at the Pentagon Tuesday that the U.S. has the capacity to evacuate 5,000 to 9,000 individuals a day now on a mix between commercial and military aircraft. However, operations may depend on whether the Taliban will allow those commercial planes to fly.

It comes as bipartisan lawmakers express concern with getting out Afghan allies who are unable to access the airport in Kabul as surrounding streets are patrolled by the Taliban.

The White House said Tuesday that both sides of the airport are now open and operational with flights able to land and depart. There were 3,500 U.S. troops on the ground to help control airport operations as of Tuesday morning.

More than 700 people were evacuated by the U.S. military on Monday, including 150 American citizens, officials said.

The Taliban's co-founder and de facto leader Mullah Baradar returned to the country Tuesday, leading a Taliban delegation that arrived in Kandahar and taking his first step onto Afghan soil in at least a decade, according to the Taliban spokesperson.

Baradar led the Taliban negotiating team in talks with the Trump administration, securing a U.S. military withdrawal and the release of thousands of Taliban prisoners -- longtime goals of the group. He remains sanctioned by the U.S. and United Nations as a terrorist, although he was granted a waiver after being released from a Pakistani prison to travel to Doha, Qatar, and participate in negotiations with the U.S.

"This afternoon, a high-level delegation from the Islamic Emirate of Afghanistan headed by Mullah Baradar Akhund left Qatar and arrived in our beloved country this afternoon and landed at Kandahar Airport," a spokesperson for the Taliban said on Twitter, translated from Arabic.

Seizing control of the country after the Afghanistan president fled, the Taliban has now claimed the formation of the "Islamic Emirate of Afghanistan."

NATO Secretary-General Jens Stoltenberg in a press conference said the North Atlantic Council has met to discuss Afghanistan, calling the situation there "extremely serious and unpredictable."

"Kabul has fallen, and the Taliban have taken control of most of the country. I am deeply saddened by what I see unfolding in Afghanistan," he said.

NATO’s focus now is to ensure the safe departure of personnel from Allied and partner countries, and of the Afghans who have helped in the mission.

Like Biden, Stoltenberg said NATO never intended to stay in Afghanistan "forever" and pointed to the "failure of Afghan leadership" for leading to the unfolding humanitarian crisis.

"Despite our considerable investment and sacrifice over two decades, the collapse was swift and sudden," he said. "There are many lessons to be learned."

Stefano Pontecorvo, NATO’s senior civilian representative to Afghanistan, said the runway at Kabul's main airport was "open" and calm on Tuesday morning.

"I see airplanes landing and taking off," Pontecorvo posted on Twitter, alongside an image and a video of the runway. "Situation under control."

The footage, which Pontecorvo took from behind a chain-link fence, showed a relatively empty runway with American troops on the tarmac and what appeared to be a military cargo plane in the distance.

Pontecorvo's tweets follow a chaotic scene on Monday when scores of Afghans rushed to the airport and onto the tarmac in a desperate attempt to leave the country, after the Taliban's takeover of the capital. Some even clung to moving planes that were trying to take off.