Mattis' resignation came soon after Trump surprised the defence establishment by announcing U.S. troops would soon leave Syria.
Shanahan is under consideration to be nominated as Trump's second defense secretary at a moment when the president is taking steps to end the counterterrorism wars that have dominated Pentagon activities since the 9/11 attacks.
Pat Shanahan, the recently installed acting secretary of defense, said he has no orders to reduce the US troop presence in the country, although officials say that's at the top of the Taliban's list of demands in exploratory peace negotiations.
On his first trip to Afghanistan since assuming office after the resignation of James Mattis in December, Shanahan said he didn't receive any orders like that, Efe news reported.
USA officials have held several rounds of talks with the Taliban in Qatar since a year ago in what is widely seen as the most serious bid yet for peace in Afghanistan since the Taliban were ousted by US-backed Afghan forces in late 2001.
Shanahan is expected to meet Afghan President Ashraf Ghani.
"Of course it has given leverage to the Taliban, there is no question about that", the official told Reuters.
Some in Afghanistan have opposed any peace deal with the Taliban, while the Taliban has so far refused to negotiate with the government in Kabul, calling government officials puppets of the West.More news: Measles making comeback on widening anti-vaccine movement
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US troops invaded in October 2001 and toppled the Taliban government within weeks after Osama bin Laden's al-Qaida network launched the 9/11 attacks from Afghanistan.
Some politicians attending Moscow talks proposed the establishment of an interim government; however, in the Moscow declaration did not mentioned anything about such a plan.
According to a statement issued later by Cmdr.
"The U.S. military has strong security interests in the region".
For years, Pentagon leaders viewed the idea of striking a deal with the Taliban with skepticism, adding to an array of obstacles that scuttled past attempts.
Khalilzad is going to lead an "interagency delegation" from the US to Belgium, Germany, Turkey, Qatar, Afghanistan and Pakistan for the rest of the month as "part of an overall effort to facilitate a peace process", a spokesperson for the US State Department said Sunday.
The statement said that "the trip is part of an overall effort to facilitate a peace process that protects U.S. national security interests and brings all Afghan parties together in an intra-Afghan dialogue through which they can determine a path for their country's future". Backed by 2,000 US troops, the Kurdish-led Syrian Democratic Forces (SDF) said they are in a final showdown against ISIS after helping to drive the group from the towns and cities that once formed its self-proclaimed caliphate in eastern Syria.
Taking a potshot at Afghan politicians who went to Moscow to hold talks with a Taliban delegation, Ghani said it serves no objective as they lack the implementation power. He says he is ready to pay the ultimate sacrifice for peace and stability in Afghanistan.
"We do not know whether we will achieve an agreement", Mr. Trump said in his State of the Union address to Congress last week, "but we do know that after two decades of war, the hour has come to at least try for peace". That uncertainly creates complications for negotiators and for military officials as they seek to reassure Afghans they will not leave overnight.