"Just finished a marathon round of talks with the Taliban in #Doha. The conditions for #peace have improved. It's clear all sides want to end the war. Despite ups and downs, we kept things on track and made real strides," Zalmay Khalilzad tweeted.
Khalilzad said the next step would be discussions in Washington, adding that "there is no final agreement until everything is agreed".
The talks focused on the withdrawal of US troops and assurances that insurgents would not use Afghanistan's territory to stage future terrorist attacks -- one of the initial aims of American policy in the wake of the 9/11 attacks back in 2001.
"Progress was achieved regarding both these issues," said a Taliban spokesman.
"For now, both sides will deliberate over the achieved progress, share it with their respective leaderships and prepare for the upcoming meeting." The Afghan government, which was not involved in the Doha negotiations, said it was pleased with the progress made.
"We hope to witness a long term comprehensive ceasefire with the Taliban and hope that direct negotiations of (the) Islamic Republic of Afghanistan and the Taliban begin soon," said Haroon Chakhansuri, the president's spokesperson, on Twitter.
On the issue of troop withdrawal, it had been previously speculated that US forces could leave Afghanistan within five years as part of a deal to end the nearly 18-year war.
The Taliban have called for the withdrawal to be much quicker, around six months.
There are some 14,000 US troops currently based in Afghanistan.
The latest meetings follow marathon talks in January that saw the US and the Taliban walk away with a "draft framework" on the two issues.
Discussions also took place between the Taliban and Afghani opposition groups in Moscow in February.
During the current round of negotiations in Doha, violent attacks in Afghanistan continued.
On March 1, at least 23 Afghan security forces were killed in a Taliban attack on a joint US-Afghan base in southwestern Afghanistan.
Analysts have warned that the Taliban are likely to ramp up attacks in the coming months as they seek to maintain momentum on the battlefield and leverage at the negotiating table.
It is unclear as yet when the next round of talks will take place. The talks which finished on Tuesday spanned 16 days, which is thought to be the longest consecutive discussions held between both sides.
They bizarrely took place in a luxurious Doha hotel village resort, where the negotiating rooms were located only metres (yards) away from a pianist and not far from a swimming pool where Western tourists bathed and drank alcohol.
'Just finished a marathon round of talks with the Taliban in #Doha. The conditions for #peace have improved. It's clear all sides want to end the war. Despite ups and downs, we kept things on track and made real strides,' Zalmay Khalilzad tweeted.