DUBAI, UNITED ARAB EMIRATES, JULY 7 -- Iranian Revolutionary Guards have fortified three strategic islands near the Strait of Hormuz, presenting new dangers for the United States next week when it begins to escort 11 reflagged Kuwaiti oil tankers through the Persian Gulf, according to well-informed gulf diplomats and shipping sources.

The Iranian move was intended primarily as a signal of support for a United Arab Emirates sheik who was temporarily overthrown in a palace coup last month, the diplomats said today. But it has strengthened Iran's military presence in the vital strait, the 36-mile-wide channel through which the Kuwaiti tankers and their U.S. Navy escorts must pass.

The sources said about 1,000 Revolutionary Guards, plus attack helicopters and speedboats, are now stationed on the islands of Abu Musa and Greater and Lesser Tunbs, which lie astride the main shipping lanes entering and leaving the strait. The Tunbs are about 40 miles west of the strait and Abu Musa is 60 miles west of it.

"We know for a fact that the Iranians have fortified their bases on the islands," said a gulf shipping source. He said he had seen heightened Iranian military activity on and around the three small islands since the middle of last month during his frequent runs through the strait.

"We've seen helicopters taking off and landing from Abu Musa and lots of Iranian gunboats moving between the islands," he said.

Western diplomats in the gulf who keep close watch on the so-called "tanker war" in the long conflict between Iraq and Iran corroborated the accounts of the shipping source. They said the Iranians had built a helicopter pad and a small airport on Abu Musa, the largest of the three islands.

Iran also has bracketed the Strait of Hormuz with three fixed sites for Chinese-made Silkworm surface-to-surface missiles, which are intended for use against ships.