WASHINGTON--The Navy, in an unexpected first step toward taking on expanded duties in the Persian Gulf, escorted a Kuwaiti merchant ship carrying U.S. arms to Bahrain on Monday, the Pentagon disclosed last night.

The Kuwaiti cargo ship was carrying American-made M-60 tanks that had been sold to Bahrain by the United States. The ship completed its journey Monday night "without incident," the Pentagon said.

The Defense Department, in a statement released in response to press queries, said it decided to offer escort protection to the vessel because "it is important that the United States remain a reliable supplier of defense items to friendly countries."

"On a case-by-case basis, given the situation in the Persian Gulf, we will escort [such] shipments to friendly non-belligerent countries," the statement added.

The disclosure came as a surprise because the Reagan administration had previously announced plans only to provide escort protection to Kuwaiti oil tankers that changed their registry to the U.S. flag. That protection is expected to begin sometime early next month, according to Pentagon sources.

The administration had said nothing before yesterday about using U.S. warships to keep an eye on foreign vessels if they were carrying American-made arms.

While the Reagan administration weighs options for protecting oil tankers in the Persian Gulf, China, in a potentially explosive move, has shipped a second delivery of Silkworm antiship missiles to Iran.

The missiles were delivered about two weeks ago, adding to an arsenal provided by China earlier in the year, a U.S. offcial said yesterday. They could enhance Iran's ability to attack ships moving through the Strait of Hormuz.

"The missiles add a dangerous dimension to shipping in the Gulf," said the official, who demanded anonymity.

He said Iran had prepared sites for the weapons, but had not deployed either batch of Silkworms.

Secretary of State George P. Shultz asked the Chinese on a trip to Beijing in March to halt weapons shipments to Iran. He reiterated the appeal to a visiting Chinese military delegation here last week.

Administration officials said China's reply was to not acknowledge arming Iran with the short-range rockets.

To keep oil flowing, the United States is preparing to put its flag on 11 Kuwaiti tankers after they are inspected by the Coast Guard. The process probably will be completed early next month, officials said.

So far this year, Iran has launched 20 attacks on ships in the gulf. At least 16 had a connection with Kuwait.

Although President Reagan and other top officials have vowed to proceed with the plan to allow Kuwaiti tankers to change their registry, the move has drawn criticism on Capitol Hill in the wake of the May 16 attack on the frigate USS Stark.

The Stark was heavily damaged in an Iraqi air attack that left 37 sailors dead. Both the United States and Iraq have described that attack as inadvertent and a case of mistaken identity.

The Stark was part of the Navy's Mideast Task Force, which normally consists of six or seven small warships that patrol the Persian Gulf to keep an eye on U.S. vessels.

Although the attack on the Stark was the first on an American vessel, the Navy's work in the gulf has become increasingly dangerous because Iran and Iraq--which have been fighting each other for almost seven years--have stepped up attacks on commercial shipping in a bid to cripple each other's economy.