Saudi Arabia has arrested 18 people it accuses of spying for an unnamed foreign country, a move expected to stoke already rising tensions with the Shia community in the kingdom’s Eastern Province.

The arrests include 16 Saudis, an Iranian and a Lebanese national and took place across Mecca, Medina, Riyadh and the Eastern Province. Activists in Saudi Arabia say the 16 Saudis were all Shia. The arrests are expected to further inflame relations between the Shia-dominated Eastern Province and the kingdom’s Sunni rulers.

The oil-rich Eastern Province has become increasingly restive over the past two years, and the government’s crackdown on protests more severe. Earlier in March, a court in Saudi Arabia sentenced two high-profile Shia activists to 10 years in prison for offences, including sedition and giving inaccurate information to foreign media.

Mohammed Fahd al-Qahtani and Abdullah al-Hamed had both called for a constitutional monarchy and an elected parliament.

A statement from the Interior Ministry issued on 19 March said “the arrested men were involved in collecting information on vital locations and installations, as well as communicating with intelligence bodies in [a] foreign country”. It added that they would be interrogated and their cases referred to the courts.

There are growing calls for reform in the kingdom. On 16 March, influential cleric Sheikh Salman al-Odah issued an open letter urging the royal family to make progress on reform or risk “a spark turn into a raging blaze”, and added “people do not want violence to become the mode of expression”.

In the letter he also said, “Many citizens fear anarchy and lawlessness. If their fears are to be placated they need to see a realistic programme of reform.”

So far Saudi Arabia’s response to the protest movement that has engulfed much of the Middle East for over two years has been to approve billions of dollars of spending on social benefits, housing and healthcare projects, while continuing to make small steps towards political reform. In January, King Abdullah’s issued a decree to appoint women to the Shura Council.