With the security situation in Afghanistan deteriorating, US President Joe Biden on Thursday defended his decision to pull military forces out of the country, saying the Afghans must decide their own future and that he didn't want to see any more Americans put at risk in an "unwinnable" war.

Speaking in the White House East Room, Biden said the Afghan military has the ability to repel the Taliban, whose advances in recent weeks have raised fears the country will slide into a civil war.

Taliban fighters seized control Thursday of a district in western Afghanistan that includes a major border crossing with Iran, Afghan security officials said, as the Taliban continued their rapid military advances around the country.

The Taliban now controls roughly a third of all 421 districts and district centers in Afghanistan and is fighting for control of 42 percent more.

In the last week, the Taliban have overrun areas bordering five countries — China, Iran, Tajikistan, Turkmenistan and Pakistan.

Biden set a date of Aug 31 for the final withdrawal of US forces, minus about 650 troops to provide security for the US embassy in Kabul.

Biden said the United States had long ago achieved its objective for invading the mountainous landlocked country at the crossroads of central and south Asia.

It was to root out al-Qaida militants and prevent another attack on the United States like the one launched on Sept 11, 2001. The mastermind of that attack, Osama bin Laden, was killed by US Navy SEALs in 2011 in Pakistan.

"The mission was accomplished in that we got Osama bin Laden, and terrorism is not emanating from that part of the world," he said.

He said the "only way there's going to be peace and security in Afghanistan is if they work out a modus vivendi with the Taliban. ... And the likelihood there's going to be one unified government in Afghanistan controlling the whole country is highly unlikely."

Russian Foreign Minister Sergey Lavrov, in a speech Thursday at the Far Eastern Federal University in Vladivostok, said that Washington has admitted defeat.

"The US is not just withdrawing its troops from Afghanistan," Lavrov said. "They are withdrawing them having de facto recognized the failure of their mission."

Lavrov said the withdrawal has complicated matters in the country because there is not a political will for a transitional government, RT.com reported. He said that the Taliban are now behaving more militantly as a result.

"Unfortunately, the leaders in Kabul are not very willing to organize something transitional," he said. "It turns out that both they and the Americans are leaving the whole situation to be solved by force."

Unannounced, the United States last weekend abandoned Bagram air base, the longtime staging ground for US military operations in the country.

The base was looted shortly after it was evacuated.

Among the items pilfered were laptop computers and gas canisters, said Darwaish Raufi, a district administrator for Bagram, The New York Times reported.

"Unfortunately, the Americans left without any coordination with Bagram district officials or the governor's office," Raufi said. "Right now our Afghan security forces are in control both inside and outside of the base."

Washington had agreed to withdraw its troops in a deal negotiated last year under former president Donald Trump.

Biden overruled military leaders who wanted to maintain a larger presence to assist Afghan security forces and prevent the country from becoming a staging ground for extremist groups.

General Austin Miller, the commander of US troops in Afghanistan, warned last week that the country may be headed toward civil war.

White House press secretary Jen Psaki said before Biden's remarks Thursday that "the question fundamentally facing him was after 20 years was he going to commit more American troops to a civil war in Afghanistan".

The US intelligence community believes the Afghan military is weak and that the Kabul government's prospects for survival short term are not good, US government sources said.

The Biden administration in recent days has worked to frame ending the conflict as a decision that the president made after concluding it's an "unwinnable war" and one that "does not have a military solution".

"How many more, how many more thousands of American daughters and sons are you willing to risk?" Biden said to those calling for the US military operation to continue. "I will not send another generation of Americans to war in Afghanistan, with no reasonable expectation of achieving a different outcome."

Answering questions from reporters Thursday, Biden said that Kabul falling to the Taliban would not be acceptable.

"Do I trust the Taliban? No," he said. "But I trust the capacity of the Afghan military, who is better trained, better equipped and more competent in terms of conducting war."

Reuters and The Associated Press contributed to this story.