Zalmay Khalilzad, the United States peace envoy to Afghanistan, is in Beijing for a previously scheduled meeting, an American Embassy spokesman said on Thursday, as indications grew of new momentum in efforts to end Afghanistan's 18-year war and push by China to boost its influence in the region.

The spokesman gave no further details about the visit by Zalmay Khalilzad, but it comes just days after he said that for the first time he can report “substantive” progress on all four issues key to a peace agreement.

The spokesman spoke on routine condition of anonymity.

Khalilzad said the latest round of talks with the Taliban had been the “most productive” yet, and had broadened to include a timeline for both intra-Afghan negotiations and a cease-fire.

US Secretary of State Mike Pompeo said last month that Washington was hopeful a peace agreement to bring an end to the war can be reached before Sept 1.

Reports in Afghanistan said a Taliban delegation and former president Hamid Karzai had also recently visited Beijing.

Along with Pakistan, Iran, Russia and China are gaining influence in Afghanistan even as the US spends billions of dollars to support the Afghan National Security Forces.

China has hosted previous rounds of talks between the Taliban and the Afghan government and has substantial interests in the country's mineral wealth, as well as in reducing the chances of the nation's violence and chronic instability flowing across the two countries' narrow border in the remote Wakhan corridor region.

China also joined in meetings with the US and Russia over the Afghan peace process and a joint statement issued by the three following talks in Moscow in April said they encouraged the Taliban to “participate in peace talks with a broad, representative Afghan delegation that includes the government as soon as possible”.

The statement also said the sides took note of the Taliban's commitment to cut ties with “international terrorist groups,” including those fighting to overthrow Beijing's rule in the far northwestern Chinese territory of Xinjiang.

It added that the Taliban should also ensure areas they control are not used to threaten any other country, prevent terrorist recruiting, training, and fundraising, and "expel any known terrorists.”

China secured rights to Afghanistan's Mes Aynak copper deposit believed to contain about 450 million tons of the metal worth tens of billions of dollars. However, the poor security and economic chaos in the country prevented the development of the mine, which also sits on an ancient Buddhist pilgrimage site.

Afghanistan's former ambassador to China, Janan Mosazai, said last year that Beijing was helping Afghanistan set up a mountain brigade to bolster counterterrorism operations, but that no Chinese troops would be stationed in the country. Along with that military assistance, China also provided equipment and training to Afghanistan's government.

Despite the denials of Chinese military activity in the area, unconfirmed reports have shown what appear to be Chinese military vehicles operating in the Wakhan corridor, which lies in the shadow of the Hindu Kush mountains with Tajikistan to the north and Pakistan to the south.

Afghanistan is also an observer member of the Shanghai Cooperation Organisation, a Beijing-led bloc, which experts see as seeking to counter American influence in Central Asia.