WASHINGTON, Sept 14 (Reuters) - The chairman of the Senate Foreign Relations Committee threatened on Tuesday to subpoena Secretary of Defense Lloyd Austin and other officials if necessary to make them testify to Congress about the chaotic U.S. withdrawal from Afghanistan.

"A full accounting of the U.S. response to this crisis is not complete without the Pentagon – especially when it comes to understanding the complete collapse of the U.S.-trained and funded Afghan military," Democratic Senator Bob Menendez said at the second congressional hearing in two days, which included testimony from Secretary of State Antony Blinken.

"I expect that the Secretary (Austin) will avail himself to the Committee in the near future. If he does not, I may consider the use of the Committee’s subpoena power to compel him and others over the course of these last twenty years to testify," Menendez said.

A Pentagon spokesman responded that Austin was unable to appear because of "conflicting commitments" and added that Austin would testify at the end of September before the Senate and House of Representatives Armed Services Committees.

Menendez told MSNBC after the hearing that he nonetheless wanted Austin to appear before the foreign policy panel.

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Lawmakers - President Joe Biden's fellow Democrats as well as Republicans - peppered Blinken with questions and criticism during the 3-1/2 hour hearing about the messy end last month to America's longest war and why the administration did not delay the withdrawal to allow more people to be evacuated.

Menendez blasted the exit as "clearly and fatally flawed."

Blinken said U.S. officials had not expected the Taliban's lightning advance and the "11-day collapse" of U.S.-backed Afghan forces.

"That's what changed everything," Blinken said.

U.S. forces had been in Afghanistan since toppling the Taliban in the wake of the Sept. 11, 2001, attacks which they say were masterminded by al-Qaeda leaders based in the country.

Senator Jim Risch, the committee's top Republican, said he worried the administration was seeking to normalize relations with the Taliban and called plans to restart humanitarian aid "deeply, deeply concerning." He described the militant group as "one of the best-armed terrorist organizations on the planet," now that it controls military equipment left behind by U.S. forces.

"There is not enough lipstick in the world to put on this pig to make it look any different than what it actually is," Risch said.

Members of Congress, which is narrowly controlled by Biden's fellow Democrats, have pledged to investigate since the collapse of the Kabul government and takeover of Afghanistan by the Taliban last month.

Representative Michael McCaul, the top Republican on the House Foreign Affairs Committee, announced after Monday's House hearing with Blinken that he had hired a former CNN reporter to investigate the withdrawal.

Blinken, and many Democrats, repeatedly noted that Republican former President Donald Trump had negotiated the withdrawal agreement with the Taliban. Several Democrats accused Republicans of hypocrisy for supporting Trump's planned withdrawal but opposing Biden's action.

(Reporting by Patricia Zengerle and Doina Chiacu; additional reporting by Humeyra Pamuk and Idrees Ali; Editing by Alistair Bell)