MANAMA, BAHRAIN, AUG. 17 -- While idled Iraqi and Kuwaiti commercial ships piled up in Middle Eastern waters with nowhere to dock, the U.S. Navy today began its interdiction program aimed at enforcing U.N. economic sanctions against Baghdad.

Two U.S. Navy ships, a guided-missile cruiser and a frigate, halted two Iraqi vessels off the coast of Bahrain but allowed them to proceed after determining that they were empty, the Pentagon reported. It said the ships were not boarded.

The program, which U.S. officials say is not a blockade but could involve the use of force, began amid international bickering over the legality of the U.S. action as well as rising pressures on some of Baghdad's most important trading partners, who face potentially disastrous economic problems if the stranglehold on Iraq continues for long. Under the program, President Bush has directed the Navy, in portions of the Persian Gulf and Red Sea, to intercept ships carrying cargo to or from Iraq or Kuwait.

Shipping sources said nearly all of Iraq's fleet of about 80 tankers and cargo vessels has been stranded at sea and many ships apparently have been told to circle idly while awaiting opportunities to dock. In addition, Kuwaiti oil tankers are among dozens of vessels anchored in the southern end of the tense Persian Gulf, with no apparent intention to move anytime soon.

Saudi Arabia today turned away an Iraqi oil tanker that tried to berth at Muajjez, Saudi Arabia, the Red Sea terminus of Iraq's trans-Arabian oil pipeline, which handled about one-third of Baghdad's oil exports before U.N. sanctions turned off the spigot. Last week, another Iraqi tanker was refused entry to Muajjez by Saudi port officials.

The Iraqi tanker turned away today was reported to have returned to a position in the Red Sea not far from Muajjez, where a Norwegian supertanker and a Bulgarian cargo vessel also have been waiting for days.

Bulgaria is one of a half-dozen economically fragile, relatively neutral countries that have barter-trading arrangements with Baghdad wherein Iraq swaps oil directly for food, minerals, arms and manufactured goods. These countries, including debt-ridden Brazil and India, have relied on barter deals with Iraq to conserve scarce foreign exchange and hold down energy costs.

U.S. officials have tried to assure vulnerable countries that the United States will do what it can to defray the impact of its enforcement of U.N. sanctions against Iraq, but the list of needy cases is long and the potential costs are enormous.

India has only enough foreign exchange on hand to cover two months' worth of imports and could see that reserve dwindle quickly if it must use cash to replace Iraqi oil supplies at higher world prices, according to Indian officials. For Brazil, too, Iraq was the most important source of oil to an economy trying to overcome hyperinflation through the radical introduction of free-market conditions.

While both India and Brazil have pledged to abide by the sanctions, their commitment might wilt under domestic economic pressures, according to government officials, diplomats and shipping executives in the gulf. Such nonaligned countries might be motivated to dodge sanctions particularly if the U.S. naval interdiction program is seen as excessive or in violation of U.N. rules.

Already there has been public questioning of the decision by the United States and Britain to forcibly intercept commercial vessels headed to or from Iraq or Kuwait. France has said it will not intercept suspected Iraqi trading vessels unless it is authorized to do so by the U.N. Security Council.

While the sanctions against Iraq authorized by a 13-to-0 vote in the Security Council do not authorize force, the Pentagon is relying on a written request from the exiled Kuwaiti government to block trade with Iraq and occupied Kuwait, as well as on Chapter 51 of the U.N. Charter, which allows member states -- in this case, Kuwait -- to use force in self-defense.

The oil-rich but militarily vulnerable gulf states appear to fully support the U.S. and British interdiction programs. "Maybe there's a slight confusion, but whatever they say in public, everyone knows the Americans have been leading the way to safeguard the international waters of the gulf," said a senior official of one gulf state. "It's nobody's idea of a picnic, but it's necessary."

With shipping insurance rates soaring, dozens of commercial vessels anchored or circling idly in the gulf and warships from several Western countries arriving to carry out missions they do not fully agree on, the mood at sea these days is both apprehensive and bewildered, according to several commercial ship captains.

One concern is whether the rules of engagement promulgated by the Pentagon Thursday will lead U.S. naval commanders to use excessive force or so bog them down in regulatory constraints that they are unable to effectively control ships attempting to break the sanctions.

According to a U.S. Navy spokesman in the gulf, the rules of engagement, which the Navy will discuss only in general terms, authorize the "minimum force necessary" to enforce the U.N. sanctions. But the spokesman said the interpretation of that phrase will depend in large part on the judgments of individual Navy commanders. "You take the science of naval warfare and combine it with the political art to come up with guidelines," said spokesman Lt. Cmdr. Ron Morse. "There are questions that are difficult to resolve, but I don't see so many of those."

Commercial ship captains in the gulf say that while they are concerned about the scope of the U.S. program, they are buoyed by their past dealings with U.S. warships that escorted Kuwaiti oil tankers through the Persian Gulf during the Iran-Iraq war.