RIYADH (Reuters) - Saudi Arabia will open a digital centre to monitor the activities of Islamic State in Iraq and Syria (ISIS) and other Islamist militant groups online, a senior Saudi official said on Saturday, as US President Donald Trump began a visit to the kingdom.

Mohammed al-Issa, secretary-general of the Saudi-based Muslim World League and director of the Defense Ministry's new Ideological War Centre, told reporters the centre would open on Sunday in an inauguration ceremony with Trump and Saudi King Salman.

Ahead of Trump's trip, the White House said the president expected tangible results from Saudi Arabia in countering Islamic extremism.