Saudi authorities have arrested 10 more suspects in an alleged Iranian spy ring unveiled two months ago, an interior ministry spokesman said on Tuesday.
The eight Saudis, a Lebanese and a Turk bring the number of people arrested to 28, the official SPA news agency quoted the spokesman as saying.
However, a Lebanese arrested in March has since been released.
"The results of investigations by security services have led to the arrest of the other 10 involved in spying for this cell," the spokesman said.
On March 19, the interior ministry said authorities had arrested 16 Saudis, an Iranian and a Lebanese in four regions, including the oil-rich Eastern Province, where Saudi Arabia's Shiite Muslim minority is concentrated.
A week later, the kingdom said the alleged spy cell had "direct links" to Iran's intelligence services.
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"Preliminary investigations and physical evidence that has been collected as well as the defendants' statements on this case have all revealed direct links between this cell and Iranian intelligence services," a ministry spokesman said.
Shiite-dominated Iran has denied any involvement in the alleged spy ring.
Saudi Arabia's ultra-conservative Sunni Muslim rulers have strained ties with Iran, which deteriorated further after a Saudi-led military intervention crushed Shiite-led pro-democracy protests in neighbouring Bahrain in early 2011.
Since early 2011, mainly Shiite towns in the Eastern Province have seen sporadic protests and confrontations between police and residents complaining of marginalisation.
There are an estimated two million Shiites in the Sunni-dominated Saudi kingdom of about 27.5 million people.
On March 27, 135 Saudi Shiites, among them 36 clerics, urged the government to free members of the alleged spy ring, accusing it of using sectarianism to settle foreign scores.