Bulgaria on Wednesday accused Iran of refusing a plane taking its prime minister to Saudi Arabia through its airspace, but the Islamic republic denied it rejected the overflight request.

Prime Minister Boyko Borisov had been heading towards Iran's main regional rival Saudi Arabia on Tuesday when his Bulgarian government aircraft was denied entry into Iranian airspace.

"The refusal is inexplicable," Bulgarian Foreign Minister Ekaterina Zaharieva was quoted by Bulgarian media as saying from Riyadh, adding that the Iranian ambassador in Sofia had been summoned for an explanation.

But Iran's foreign ministry spokesman Bahram Ghasemi told AFP it was "a simple technical problem".

"The Bulgarian embassy provided the coordinates of the flight and we gave authorisation, but the coordinates transmitted by the pilot to the control tower did not correspond with those which had been provided, so they refused the overflight," said Ghasemi.

"There was no political intention. We have even given permission for the return flight," he added.

The plane was carrying Borisov, Zaharieva, former Bulgarian king Simeon Saxe Coburg and four other ministers.

It was the first visit by a Bulgarian prime minister to Saudi Arabia since the former communist country, now a member of the European Union, established relations with the kingdom 20 years ago.

Borisov was due to be received on Wednesday by King Salman and by the powerful Crown Prince Mohammed bin Salman, under whom relations with Iran have deteriorated dramatically.