U.S. diplomats have spoken to about 500 of those people and are “aggressively” trying to reach the others, Blinken said at the State Department on Wednesday. “We continue to be relentless in our outreach,” he added.

Blinken said U.S. officials have been hamstrung because not all Americans who traveled to Afghanistan over the past two decades registered with the embassy in Kabul. Others never removed their names from the embassy registry and the State Department must determine how many are actually American citizens, he said.

A majority of the Americans believed to still be in Afghanistan may not be seeking to leave, Blinken said, citing the case of people who hold dual nationality.

The top U.S. diplomat’s remarks come after Biden administration officials resisted calls for days to disclose how many U.S. citizens remained in Afghanistan with President Joe Biden’s Aug. 31 withdrawal deadline approaching. In recent days, officials from both the State Department and the Pentagon would only say that “several thousand” remained.

Blinken said about 4,500 Americans have been evacuated from Afghanistan since Aug. 14. He said the U.S. is “developing detailed plans” for diplomatic options to assist departures after Aug. 31.

“This effort does not end on Aug. 31,” Blinken said, saying the U.S. will use every economic and diplomatic tool it has in dealing with the Taliban.

Congressional Demands

Biden has resisted calls from members of Congress and U.S. allies to extend the end-of-August deadline, but he said Tuesday that he ordered his national security team to come up with contingency plans if more time is needed. He said the risks to American troops is growing the longer they stay in the country and expressed confidence that the U.S. could complete it’s mission by the end of the month.

Fears for people whose past ties to the U.S. and other Western powers leave them vulnerable are growing after more reports of reprisals and a threat by the Taliban to stop Afghans from traveling to the airport.

The Pentagon said Wednesday that U.S. military aircraft evacuated 11,200 people from Afghanistan in the previous 24 hours. An additional 7,800 people departed on allied aircraft, Army Major General William Taylor told reporters. Since evacuations began, about 88,000 people have been brought out of Afghanistan by the U.S. and allies, Taylor said.

Members of Congress from both parties renewed their concerns about Biden’s Aug. 31 deadline and the prospect that interpreters and others who worked with the American military will be left behind.

“Will partners in the future be able to trust the United States of America?” Representative Michael McCaul, the top Republican on the House Foreign Affairs Committee, said at a news conference on Wednesday.

Joining him, Democratic Representative Andy Kim of New Jersey, said, “What this comes down to is what is the value of the American handshake.”