The Democratic chairman of the House Foreign Affairs Committee on Thursday issued a subpoena to President Donald Trump's diplomat negotiating with the Taliban, demanding the envoy testify before lawmakers about his talks with the insurgents.

Rep. Eliot Engel of New York accused the Trump administration of keeping Congress and the American people "in the dark" about the negotiations, which are aimed at opening the way to the withdrawal of U.S. troops from Afghanistan after 18 years of war.

The subpoena reflects growing frustration in Congress on both sides of the aisle about the administration's reluctance to discuss the negotiations at an open hearing or in private.

In nearly a year of negotiations, the envoy, Zalmay Khalilzad, has briefed lawmakers on the talks only once, when he met behind closed doors with senators in May. He has not briefed lawmakers in the House.

Engel said the subpoena requires Khalilzad to testify before the House committee on Sept. 19 at 10 a.m. "sharp."

The State Department did not immediately respond to a request for comment. Khalilzad, a former ambassador to Afghanistan who has rarely spoken publicly about the talks, was not immediately available for comment.

As U.S. officials prepared to clinch a proposed U.S.-Taliban deal over the weekend, President Trump called off negotiations with the Taliban. He also canceled a meeting he said he had planned with Taliban representatives and the Afghan government at the Camp David presidential retreat.

On Monday, he said the talks with the insurgents were "dead."

Engel said he issued the subpoena because he said the State Department failed to respond to numerous requests for briefings by the presidential envoy.

"More than 2,000 American troops have died in Afghanistan, and I'm fed up with this Administration keeping Congress and the American people in the dark on the peace process and how we're going to bring this long war to a close," Engel said in a statement.

Rep. Eliot Engel, D-NY, speaking at a press conference. Michael Brochstein/SOPA Images / LightRocket via Getty Images file

"For months, we haven't been able to get answers on the Afghanistan peace plan, and now the President is saying the plan is dead," Engel added. "We need to hear directly from the Administration's point person on Afghanistan to understand how this process went off the rails."

Engel said the State Department declined requests from his committee in February, April and earlier this month.

Before Trump called off the talks, the proposed deal between the United States and the Taliban outlined a timetable for the phased withdrawal of U.S. troops in return for the militants agreeing to hold peace talks with the Afghan government and promising to prevent the country from being used as a staging ground for terrorist attacks, according to current and former U.S. officials and foreign diplomats.