* A third U.S. national has been detained since Nov. 2017
* New arrests come amid trial that has sharpened Western criticism
* Kingdom's rights record in the spotlight after Khashoggi murder (Adds U.S. statement)
DUBAI, April 5 (Reuters) - Saudi Arabia has arrested eight people, including two dual U.S.-Saudi citizens, in an apparent crackdown on supporters of women activists whose trial has drawn Western condemnation, rights groups said on Friday.
The 11 women on trial had campaigned for the right to drive and an end to the kingdom's male guardianship system. Their case has intensified criticism of Riyadh's rights record, already in the spotlight after last year's murder of journalist Jamal Khashoggi.
The U.S. nationals are journalist Salah al-Haidar, whose mother Aziza al-Yousef is among those on trial, and Bader al-Ibrahim, a doctor and author of a book about Shi'ite Muslim politics, the associate and London-based Saudi rights group ALQST said.
Those newly detained, seven men and one woman, are not frontline activists but have expressed support of women's rights and other reforms. They were detained late on Wednesday and Thursday, according to ALQST and an associate, who spoke on condition of anonymity.
Five other people close to the women have also been placed under a travel ban since February, they added.
An official from the U.S. State Department confirmed the arrest of two U.S. citizens and said it had "engaged the Saudi government in this regard", without providing details.
The Saudi government communications office and the U.S. Embassy in Riyadh did not respond to requests for comment.
Lynn Maalouf, Amnesty International's Middle East research director, said the authorities were "signalling to their entire people that there will be zero tolerance of any form of criticism, let alone questioning, of the state's authoritarian practices."
A Riyadh court last week temporarily released three of the women on trial, including Yousef, raising hopes of a more lenient handling after months of lobbying by Western governments.
But the new arrests could signal that the authorities will resist international pressure and pursue harsh sentences.
Khashoggi's murder tarnished the reputation of Crown Prince Mohammed bin Salman, who courted Western support for his efforts to modernise Saudi Arabia. The CIA and some Western countries believe he ordered the killing, which Saudi officials deny.
Dozens of other activists, intellectuals and clerics have been arrested separately in the past two years in an apparent bid to stamp out opposition.
The women on trial were arrested last May and branded as traitors. At least one of them has been charged under the kingdom's cybercrime law and faces up to five years in prison.
Five men arrested at the same time are not on trial. Rights groups say two of them have been released, but the others' status is unclear.
Another U.S.-Saudi national, Walid al-Fitaihi, has been detained since 2017 under Riyadh's anti-corruption campaign. His son told U.S. senators last month he had been tortured in detention, including electric shocks and whipping.
Some of the women activists have also alleged torture and sexual assault, which the authorities deny.
The siblings of one woman, Loujain al-Hathloul, who have publicised her case in U.S. media, say men describing themselves as "close to the state" had asked his parents to stop them from speaking out.
"We stayed silent for eight months. We thought that being silent would solve the issue," Walid al-Hathloul told CNN on Thursday.
"We found out at the end of the day that this made the case even worse and that's why we're speaking out now. At the end of the day we didn't have any options but to speak out." (Reporting By Dubai newsroom, Editing by Raissa Kasolowsky, William Maclean)
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