The US plans to keep troops in northern Syria indefinitely to prevent the resurgence of the Islamic State of Iraq and the Levant (Isil) and counter the influence of Iran, Rex Tillerson, the US secretary of state, has said.

Laying out the Trump administration’s broad strategy for Syria, Mr Tillerson said a quick withdrawal from Syria would give the jihadist group space to regroup after years of defeat on the battlefield. 

“The United States will maintain a military presence in Syria focused on ensuring Isil cannot re-emerge,” he said. “We cannot make the same mistakes that were made in 2011 when a premature departure from Iraq allowed al-Qaeda in Iraq to survive and eventually morph into Isil”.

Mr Tillerson said the US remained committed to seeing Bashar al-Assad, the Syrian leader, step down from power and would refuse to normalise diplomatic relations with Syria or provide reconstruction funds as “tools of pressure” to get Assad to go.

But America’s top diplomat also called for “patience in the departure of Assad and the establishment of new leadership”, a signal that the US does not expect the Syrian leader, who has survived nearly seven years of civil war, to relinquish power any time soon. Boris Johnson, the Foreign Secretary, called Mr Tillerson’s speech on Syria a “great statement of US leadership”.

The US has 2,000 troops in Syria and the open-ended commitment announced by Mr Tillerson means they will remain deployed in a place of fiendishly complex and shifting alliances between different actors in the Syrian war. 

That complexity has been on display this week as America’s Nato ally Turkey has threatened to launch a full-scale assault on America’s Kurdish allies in northern Syria. 

Turkey has been enraged by US plans to help train what American officials initially described as a 30,000-strong “border force”, composed mainly of Kurdish fighters, to maintain security on the Syrian-Turkish border.  

Turkey, which is wary of any sign of the Kurds setting up their own state in Syria, accused the US of "creating a terror army” and threatened to attack the Kurdish enclave of Afrin in response.Mr Tillerson hastily tried to lower the tension, saying the situation has been “misportrayed and misdescribed”. “We are not creating a border security force at all,” he said.

Turkish leaders said they were still not satisfied with Mr Tillerson’s response and called on the US to “eliminate the confusion”. Turkish forces have shelled Afrin but have yet to launch an all-out attack. 

Western diplomats said the US had recently begun to engage more seriously on the future of Syria as the war winds down. 

One of the immediate priorities of both the US and the UK is to try restore the primacy of the UN-brokered Geneva peace talks between the Syrian regime and opposition over over a set of Russian-organised talks in Sochi. 

The Geneva talks have for years failed to yield any real dividends but western diplomats are concerned that the Sochi talks, dominated by Vladimir Putin, will only strengthen the Assad regime. 

But the Syrian opposition is sceptical the US will be able to bring the regime to the negotiating table given its limited leverage in Syria compared to Russia’s deep involvement in the conflict.