In an action that could increase tensions between Iran and Arab monarchies, two Iranian ships in the Persian Gulf carrying Shiite activists to Bahrain were turned back Monday by warships belonging to a coalition of gulf states that is aiding the island kingdom in its crackdown on anti-government demonstrators, according to the activists’ Web site.

The Shiite activists, members of the Islamic Revolution Supporters Association, said the Iranian government did not prevent them from sailing. But halfway to Bahrain, they decided to return to Iranian waters because of “the emergence of threats from the ships of the Peninsula Shield Force and the possibility of attacks,” the Hameyema Web site stated Monday.

The force is a military collaboration by the Gulf Cooperation Council, which consists of Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates. Bahrain’s Sunni royal family called for its help in March, after mass protests led mostly by Shiite Muslims. Saudi Arabia sent tanks and about 1,000 troops to Bahrain, assisted by police from the United Arab Emirates, as Bahraini forces led a crackdown.

The council has stationed troops in the kingdom in what it calls an effort to counter threats from Iran, and Kuwaiti warships have been assisting in sea patrols around the island. Iran, which is almost entirely Shiite, condemned the council intervention and denied involvement in the protests in Bahrain.

On Sunday, Bahraini officials condemned the Iranian activists’ plan to send the “sea caravan.”

“This would be a blatant interference in Bahrain’s internal affairs,” Sheik Fawaz bin Mohammed al-Khalifa, president of the Bahraini Information Affairs Authority, told the al-Arabiya television network. “Bahrain did not ask for humanitarian aid from the Iranian republic.”

The incident in the Persian Gulf comes after weeks of growing tensions between Iran and the Arab monarchies. On Saturday, a thaw seemed possible when Iranian Foreign Minister Ali Akbar Salehi said he welcomed a speech by Bahrain’s King Hamad bin Isa al-Khalifa, who spoke of “brotherhood” and “offering friendship” to Iran.

After a three-day publicity tour in which they met with Iranian clerics and officials, the Shiite activists boarded the two vessels and left the port of Dayyar on Monday morning.

In addition to bringing aid, an organizer told Iran’s semiofficial Fars News Agency, the ships and the 150 activists — “doctors, clerics and women with babies” — planned to “bring a message of solidarity with the oppressed and tyrannized people of Bahrain.”

Human rights groups say that dozens of Bahrainis have died in clashes and in prisons. Hundreds reportedly have been arrested, including dozens of doctors and nurses accused of treating wounded protesters.

Special correspondent Kay Armin Serjoie contributed to this report.