BEIRUT - Iran's Revolutionary Guards have detained a Saudi Arabian fishing boat and arrested its crew, an Iranian state news agency reported on Saturday, at a time of increased diplomatic tension between the two regional powers.

Five Indian nationals on the vessel were detained on Friday after they crossed into Iranian territorial waters in the Gulf, Ardeshir Yarahmadi, a spokesman for the fisheries department of Bushehr province, was quoted as saying by the Islamic Republic News Agency (IRNA).

Yarahmadi said it was the second time in the past month that a Saudi boat and its crew had been detained.

Relations between Iran and Saudi Arabia are at their worst in years, with each accusing the other of subverting regional security and supporting opposite sides in conflicts in Syria, Iraq and Yemen.

Riyadh, along with other Arab governments, has severed ties with Qatar, citing its support of Iran as one of the main reasons for the move.

Iran has blamed Saudi Arabia for being behind deadly twin attacks on June 7 in Tehran claimed by Islamic State. At least 18 people were killed and more than 40 wounded in the attacks, in which Riyadh has denied any involvement.

Iranian media reported last month that Saudi border guards had opened fire on Iranian fishing boats in the Gulf, killing a fisherman and arresting three others.

Iran has urged Saudi Arabia to release the trio, who Tehran said had legal documents but lost their way.

The Saudi Information Ministry said it had intercepted the boat in Saudi waters on June 16 and that three members of Iran's elite Revolutionary Guards had been detained in the incident.

It said the seized vessel was carrying explosives and those captured intended to conduct a "terrorist act" in Saudi territorial waters. Iran has denied the allegation.

IRAN REJECTS US THREATS OVER

DETAINED AMERICANS

Iran's government rejected US demands to release detained Americans on Saturday, saying it had no control over the judiciary.

"The judiciary, courts and judges in Iran are completely independent, as in any other country," said foreign ministry spokesman Bahram Ghasemi in a statement.

"Any interventionist and threatening statement by American officials and institutions has no effect on the will and determination of the country's judicial system to try and punish criminals and violators of the country's laws and national security," Ghasemi added.

He was responding to a White House statement on Friday, which said: "President (Donald) Trump is prepared to impose new and serious consequences on Iran unless all unjustly imprisoned American citizens are released and returned."

It followed the 10-year sentence for espionage given to a Chinese-American researcher from Princeton, Xiyue Wang, earlier in the week.

"As announced to American officials several times, the person called Robert Levinson travelled to Iran many years ago and Iran holds no new information on his fate after he left Iranian territory," Ghasemi said.

He also criticised the jailing of several Iranians by the United States in recent years on "baseless and unfounded grounds".

Washington and Tehran severed diplomatic relations in 1980 when US embassy staff were taken hostage for 444 days.

Trump has taken an aggressive approach to Iran, but has so far stopped short of tearing up the nuclear deal that eased sanctions on the country, as he had threatened on the campaign trail.

Friday's statement said Trump and his administration were "redoubling efforts" to bring back all Americans unjustly detained abroad.

An Iranian court sentenced 46-year-old Siamak Namazi and his 80-year-old father Baquer Namazi to 10 years in prison each on charges of spying and cooperating with the United States.

Iran's Islamic Revolutionary Guard Corps detained Siamak in October 2015 while he was visiting family in Tehran, relatives said.

An Iranian court sentenced Xiyue Wang, a Chinese-born US citizen graduate student from Princeton University, to 10 years in jail on spying charges, Iran's judiciary spokesman said on Sunday.

"Iran is responsible for the care and wellbeing of every United States citizen in its custody," the White House said in the statement.

Separately, Secretary of State Rex Tillerson met with Oman's foreign affairs minister, Yusuf bin Alawi on Friday.

Washington has in the past sought Oman's mediation to help in securing the release of detained Americans abroad. Last year American prisoners held captive by Yemen Houthi rebels were released after Omani mediation.  Oman also paid bail that ultimately helped in the release of three American hikers in 2010 and 2011.