Iranian boats have been landing parties of men, assumed to be infiltrators, along the United Arab Emirates (UAE) coast, according to Western sources. The incursions have taken place amid increased tension between Washington and Tehran over Iran's allegedly destabilising activities.
Oil-industry technicians operating off Abu Dhabi and local security forces have detected fast vessels making an approach at night, apparently to put ashore small groups.
The pattern of navigation is different from the normal busy dhow traffic plying the waters between the ports of southern Iran and the emirates on the opposite side of the Gulf, Western sources said. The long, deserted littoral from the border of Qatar to the Strait of Hormuz is impossible to make fully secure.
Iran is embroiled in a territorial dispute with the UAE over several small islands in the Gulf and has positioned surface-to-air missiles and other weapons in the area.
Tehran has traditionally enjoyed amicable relations and prolific trade with the Emirates but is taking a newly assertive stance towards its pro- Western Arab neighbours.
The radical Speaker of the Iranian parliament recently warned the countries of the Gulf that Iran "guarantees your security and stability".
"You should, therefore, not provide the excuse for aliens to come to the region and create a market for the sale of their weapons by creating an adverse climate." The UAE, Kuwait and Saudi Arabia have concluded big arms deals with Britain, France and the United States since the 1991 Gulf war.
The Iranian spiritual leader, Ayatollah Ali Khamenei, this week issued an incendiary call for political demonstrations "disavowing infidels" at the forthcoming haj, or pilgrimage, to the holy shrines of Saudi Arabia.
The Iranian regime has also voiced support for Shia Muslim opponents of the ruling dynasty in Bahrain, where riots and demonstrations are continuing, although Western business people and diplomats blame domestic political repression for the trouble.
The US has mounted a determined campaign to increase economic pressure on Iran, proposing an embargo on Iranian oil sales, banning US citizens from work in the Iranian petrochemical industry and seeking to stop a deal by which Russia would sell nuclear reactors to Tehran.
The White House has said it is drafting a plan to impose new sanctions on Iran that would include moves to prohibit the purchase of Iranian oil by US companies. Big American oil companies such as Exxon, Texaco, Mobil and Caltex are all involved in deals estimated at between 574,000 and 650,000 barrels per day (bpd). The purchases contribute at least $4.25bn (£2.6bn) a year to Iran's beleaguered economy and evade existing US legislation because the oil is bought and sold outside the US.
A cut-off of these purchases would probably have little effect on world oil prices but could cost Iran valuable foreign exchange in the short term. Iran produces about 3.5million bpd, of which it exports about 2.5 million.
The author Salman Rushdie has told British officials he is keen to explore a French idea of seeking assurances about his safety from the Iranian government.
The formula could be modelled on an agreement between Denmark and Iran under which Tehran undertakes not to take any measures to carry out the death "sentence'' which was imposed on Mr Rushdie by the late Ayatollah Khomeini .
Iran's clerical leaders maintain Khomeini's fatwa, or religious ruling, cannot be revoked for theological reasons. Any official agreement to ignore it, however, would still leave the field open to individual zealots keen to fulfil its provisions.