U.S. Secretary of State Mike Pompeo, on a surprise visit to Afghanistan on Monday, promised support for President Ashraf Ghani's bid to start peace talks with the Taliban and repeated the United States itself would be willing to join the talks.

The visit, coming at the end of a tour of Asian countries including North Korea and Vietnam, was Pompeo's first to Afghanistan since he became Secretary of State in April.

He said the strategy announced last year by U.S. President Donald Trump of sending more troops to increase battlefield pressure on the Taliban and push them towards negotiations "sends a message to the Taliban that they cannot wait us out".

Pompeo's visit follows one by the State Department's top diplomat for South and Central Asia, Alice Wells, who said this month that pressure was building on the Taliban to respond to Ghani's offer for peace talks.

Standing with him at a news conference in the presidential palace in Kabul, Ghani, who earlier this year offered peace talks without preconditions, said it would be necessary to move with caution.

"If we think only in one day a 40 year-crisis can be ended we are being unrealistic," he said.

Following a three-day ceasefire during last month's Eid holiday, the Taliban, fighting to restore their version of strict Islamic law, have so far rejected Ghani's offer of talks, demanding the withdrawal of international troops from Afghanistan.

However Pompeo repeated an offer for the United States to take part directly in talks with the Taliban, who have rejected talks with what they consider an illegitimate Western-backed government in Kabul and demanded direct talks with Washington.

Pompeo said the peace process would be Afghan-led but added that the United States would be prepared to participate to help resolve differences and said support from neighbouring countries would also be needed.

"We have seen good signs from lots of regional partners," he said.

As well as the battle against the Taliban and Islamic State (IS) militants operating from Afghanistan, Pompeo discussed plans for October elections in Afghanistan and presidential elections due early next year, amid tensions between powerful regional leaders and Ghani.

He said he hoped for a reduction in violence before the elections, which the Taliban have refused to support.