Riyadh, Saudi Arabia ? Saudi police detained three suspected al-Qaida members who were plotting to hijack a plane, possibly to use in a suicide attack reminiscent of the Sept. 11 terror attacks, police officials and diplomats said Wednesday.

The report — which was denied by the Saudi interior minister — coincided with the release of a new audiotape purportedly by the No. 2 man in al-Qaida, Ayman al-Zawahri, calling for new Sept. 11-type attacks.

“Consider your 19 brothers who attacked America in Washington and New York with their planes as an example,” said the speaker on the tape, excerpts of which were aired by the Arab satellite station Al-Jazeera.

The voice resembled al-Zawahri’s, judging from previous audiotapes and videotapes attributed to the Egyptian militant and key aid to Osama bin Laden. A U.S. official in Washington, speaking on the condition of anonymity, said it was plausible that the speaker was al-Zawahri, but a thorough technical analysis is necessary before authorities can be certain.

The report of the alleged hijacking plot came as Saudi Arabia was on alert for new terror attacks after May 12 suicide car bombings at three residential compounds in Riyadh killed 34 people, including nine attackers.

Saudi security officials said Wednesday that three detained Moroccans had been plotting to hijack a plane headed from the western Saudi port of Jiddah to Sudan. The officials did not say if there was a plot to use the plane as a missile, as al-Qaida did in the Sept. 11 attacks.

Later Wednesday, state television interrupted its broadcast to quote the interior minister, Prince Nayef, as saying the hijack report had “no basis in truth.” Nayef, who is in charge of police, said only two Moroccans wanted for unspecified “previous security issues” were arrested.

However, Australian Foreign Minister Alexander Downer said after meeting Foreign Minister Prince Saud that the prince had told him that three men were arrested in Jiddah.

Downer said that Saud had told him that “May 12 has been the Saudi equivalent of Sept. 11 and it has really harnessed the energy of the country to stop terrorism.”

“It was in that context that he (Saud) mentioned that they had recently arrested three people in Jiddah,” Downer told reporters. “He did say that three people had been arrested (in Jiddah) and he used that as an illustration of how the people of Saudi Arabia have been stung into action by what happened on the 12 of May.”

The discrepancy between two and three arrests could not immediately be explained.

The Saudi security officials said the Moroccans were arrested Monday amid a sweep following the Riyadh bombings, but it was unclear whether investigators believe the three were connected to those attacks. Four other suspects were in custody for the car bombings in the capital.

Nawaf Obaid, a private Saudi oil security analyst with close contacts to the Saudi government, said the three were part of a larger cell that was “in the process of carrying out suicide attacks against landmarks in the kingdom.”

Authorities have linked the Riyadh attacks to al-Qaida, though Saudi Crown Prince Abdullah cautioned in a newspaper interview that he had no solid information about a connection.

The new audiotape attributed to al-Zawahri lashed out at Arab governments that helped the United States in the war on Iraq, mentioning, among others, Saudi Arabia.

It also called for attacks on Jews and American interests. “Oh Muslims, take your decision against the embassies of America, England, Australia and Norway, their interests, their companies and their employees,” the speaker said. “Turn the earth under their feet into fire.”

Britain was the United States’ main partner in the war on Iraq, and Australia contributed troops. Norway did not take part in the Iraq fighting, but provided special forces and other support in the war that dislodged al-Qaida from Afghanistan.

The Australian foreign minister told reporters in Riyadh that “it will take a long time to finish off al-Qaida. There is an al-Qaida threat, there have been al-Qaida attacks recently and al-Qaida still exists.”

Saudi leaders and those of the other five members of the Gulf Cooperation Council met in Riyadh and called for “intensifying international efforts to fight and eradicate terrorism.”

Saudi Arabia was hardening security after warnings of new terror plots, and U.S. authorities raised the national terror alert level.

Britain, Germany and Italy joined the United States in closing diplomatic offices in Saudi Arabia for at least a few days starting Wednesday. Police patrolled Riyadh in camouflaged vehicles Wednesday and erected concrete barriers in front of major hotels.

Al-Hamra compound, one of the three hit May 12, was heavily fortified, with a police checkpoint on the approaching road about 50 yards from the main gate. Cars entering the compound had to first wend their way through massive concrete blocks and stop for security officers to check their trunks, under their hoods and scan the underside of the car with mirrors. Police armed with semiautomatic weapons stood watch nearby as drivers and passengers were questioned.

A Saudi official said Tuesday that investigators were aware of about 50 militants, some now dead, believed to belong to three Saudi terror cells, including the one that carried out the May 12 bombings. Another cell has fled Saudi Arabia and the third is at large in the kingdom, the official said.

The official indicated the survivors were ready to volunteer for more suicide strikes, were tied to al-Qaida and had hard-core sympathizers numbering “in the low hundreds.”

Investigators from the FBI and other U.S. agencies were helping investigate the May 12 bombings. CIA Director George Tenet paid a brief visit Riyadh on Tuesday, a U.S. Embassy official said. He declined to say whom Tenet met and what they discussed.