Saudi Arabia's King Salman invited Qatar's ruler Emir Sheikh Tamim bin Hamad Al-Thani to visit the kingdom, the emir's office said Monday, the latest sign of improving relations between the former rivals.

Riyadh and allies broke off ties with Qatar in June 2017 over claims it was too close to Iran and backed radical Islamist groups, allegations Qatar has always denied.

But in January, the boycotting countries agreed to reestablish ties with Qatar following a flurry of diplomatic activity by former US president Donald Trump's administration.

The emir received a letter from King Salman "including an invitation to visit" Saudi Arabia, the emir's office said in a statement.

It did not say if Sheikh Tamim had accepted or when the visit would take place.

Sheikh Tamim last travelled to the kingdom in January for the summit, hosted by the king's son Crown Prince Mohammed bin Salman, that led to Doha being readmitted to the regional fold.

He did not meet the king at that time.

The latest invitation was delivered by Saudi Foreign Minister Prince Faisal bin Farhan, who is in Doha for meetings with his Qatari counterpart.

The two discussed "Gulf unity", according to Qatar's foreign ministry. 

Since the reconciliation there have been cautious steps towards normality, including the resumption of air travel between the former adversaries and the reopening of Qatar's sole land border with Saudi Arabia.