Donald Trump has imposed sanctions against senior members of Turkey's government in response to their offensive against the Kurds in Syria, and will keep "a small footprint" of US troops in the country.

The US president said that steel tariffs would be increased to 50 per cent and negotiations over a $100 billion trade deal would be put on hold.

He signed the executive order imposing the sanctions on Monday night.

He said: "The situation in and in relation to Syria, and in particular the recent actions by the Government of Turkey to conduct a military offensive into northeast Syria, undermines the campaign to defeat the Islamic State of Iraq and Syria, or ISIS, endangers civilians, and further threatens to undermine the peace, security, and stability in the region, and thereby constitutes an unusual and extraordinary threat to the national security and foreign policy of the United States."

Earlier, in a statement announcing the forthcoming sanctions, Mr Trump said: "I have been perfectly clear with President Erdogan: Turkey’s action is precipitating a humanitarian crisis and setting conditions for possible war crimes.