It comes after Khalilzad confirmed to media this week that Washington and the Taliban had a "draft framework" in place for a deal which could pave the way for the insurgents to hold peace talks with Kabul.

The development was described by experts as a "milestone" in the grinding 17-year Afghan conflict, but prompted concerns from Afghans and observers who feared it could see US troops withdraw from the country before a lasting peace deal is reached with the government in Kabul.

"The path to peace doesn't often run in a straight line," Khalilzad tweeted, adding that the situation was "complex".

"We made significant progress on two vital issues: counter terrorism and troop withdrawal.

"That doesn't mean we're done. We're not even finished with these issues yet, and there is still work to be done on other vital issues like intra-Afghan dialogue and a complete ceasefire."

Sceptics have "rushed to judgement", he said, but added that "you can't eat an elephant in one bite!"

"And a forty year old war won't be resolved in one meeting, even if that meeting runs for close to a week."

The envoy was referring to negotiations in Qatar last week which raised expectations of a breakthrough and came after a months-long diplomatic push with the insurgents.

Afghanistan has suffered near constant conflict since the Soviet invasion of 1979, which was followed by civil war, the Taliban regime, and the US invasion.

Khalilzad stressed that the conflict has many actors with many interests, but repeated earlier calls to seize the momentum.

"This is a moment for Afghans to begin to heal old wounds and chart a new course for their country," Khalilzad said.

"We are on the right path, together. As @POTUS said, talks are proceeding well," he added, referring to US President Donald Trump.