A US negotiator on Friday voiced understanding for Afghanistan's release of Taliban prisoners who killed French and Australians, saying the "big picture" was to end the war.

France and Australia both objected to the prisoners' release, which has been pushed by US President Donald Trump's administration as the Taliban made the freedom of hundreds of fighters a condition to meet with Afghanistan's government.

After Kabul overcame hesitation and released the militants, the two sides will open landmark talks in Doha on Saturday in the presence of US Secretary of State Mike Pompeo.

"I know that none of us are happy about the release of prisoners that committed violence against forces but we want to keep the big picture in mind," said Zalmay Khalilzad, the US negotiator on Afghanistan.

The start of the talks can ensure that Afghanistan "never again becomes a threat to any of us," he told reporters.

"None of the countries that are concerned have made this an issue that will affect relations with Afghanistan," he said.

"They don't like it but at the same time they understand that this was an Afghan decision -- a decision that was difficult but necessary, they felt, at the end to start intra-Afghan negotiations and to give peace a chance."

Two Taliban prisoners who murdered Frenchwoman Bettina Goislard, a UN refugee worker, were released in the province of Wardak.

Six other militants including two who killed French and Australian soldiers were transferred to Doha on a special plane reportedly to be held in detention there.

France's foreign ministry last month said it was "firmly opposed" to the release of individuals sentenced for crimes against its nationals.

Pompeo arrived for the Afghan talks on the anniversary of the September 11, 2001 attacks, which led the United States to topple the Taliban regime that had welcomed Al-Qaeda.

Trump, who faces elections in less than two months, is eager to fulfill promises to end America's longest war and his administration negotiated a troop withdrawal with the Taliban.