The United States has called for the Taliban to "step up" and engage the Afghan government in peace talks to end the 16-year conflict.

Alice Wells, a U.S. principal deputy assistant secretary of state in charge of South and Central Asian affairs, said Afghan President Ashraf Ghani's offer of talks with the Taliban without any preconditions was the "most specific and forward-leaning proposal" for peace since the U.S.-led invasion in 2001.

Wells, speaking at the U.S. Institute of Peace in Washington on March 9, urged the Taliban to give Ghani's offer "serious consideration," adding that there were signs the militant group was "assessing the proposal."

On February 28, Ghani offered to allow the Taliban to establish itself as a political party and said he would work to remove sanctions on the militant group, among other incentives, if it joined the government in peace negotiations.

In return, the militants would have to recognize the Kabul government and respect the rule of law.

But the Taliban has continued to reject direct peace talks with the Kabul government and insisted it will only negotiate with the United States, which it calls a "foreign occupying force." The Taliban also says that NATO forces must withdraw before negotiations can begin.

The United States has refused to withdraw troops as demanded by the Taliban and has insisted that the Kabul government must play a lead role in peace negotiations.

Wells, who is leading the Afghan peace effort for the U.S. government, said Washington was supporting talks without preconditions to allow for "creative solutions and approaches."

She also said Washington had not seen "decisive and sustained changes from Pakistan," which Afghan and U.S. officials have accused of supporting the Taliban.

"But we won't walk away from Pakistan," Wells said, adding that Islamabad had a crucial role to play in bringing the militants to the negotiating table.

Russia and Iran have established contacts with the Taliban in recent years because of the common threat posed by the Islamic State (IS) militant group in Afghanistan.

But Wells said some regional countries were "exaggerating" the threat of IS militants in Afghanistan as a "pretext for hedging behavior."