Trump pledges help for Turkey on Syria

US President Donald Trump has congratulated Turkish President Tayyip Erdogan on his referendum campaign in a phone call and vowed close co-operation on Syria, Iraq and the fight against Kurdish militants, Turkish presidential sources say.

"I value our friendship, we have many important things to do together," Trump was quoted by the sources as saying in a statement giving a readout of the call.

Turkey voted on Sunday to switch to a presidential system, greatly increasing Erdogan's powers.

Unofficial results, which the opposition said it would challenge, showed a narrow victory for him with 51.4 per cent of votes cast in favour.