Saudi King Salman has begun weighing in to try to defuse the growing crisis over missing journalist Jamal Khashoggi, as the kingdom goes on the offensive triggering a fierce online media campaign.

In a phone call with US President Donald Trump on Monday, the king denied "any knowledge of whatever may have happened 'to our Saudi Arabian citizen'", the US leader said in a tweet. Trump added he was "immediately" sending Secretary of State Mike Pompeo to Riyadh "to meet with the king" for talks on the crisis.