The United States and the Soviet Union concluded a second round of talks on bilateral shipping issues in Kings Point, N.Y., Friday.

The five days of discussion at the U.S. Merchant Marine Academy were a follow-up to talks in Moscow in January.Industry observers believed a third round will be needed to iron out differences thatremain, apparently on port access, access to United States cross trades and cargo reservation provisions.

Members of the U.S. delegation did not return telephone messages left for them at the academy Friday morning.

A spokeswoman for the academy said the U.S. and Soviet negotiators concluded their business around noon Friday and then left the area.

There were unconfirmed reports Friday that the Soviet delegation dropped a counterproposal on the Americans late Thursday. No details on the new proposal, if indeed there was one, were available.

The negotiators, meeting at Mr. Leback's alma mater, followed a tightly controlled agenda and away from public scrutiny. One maritime industry source said the "prime players were locked up all week."

Another source said there was "still too much up in the air" to have expected a final agreement at this time.

Reports from several sources indicated that, while the Soviets are not fully

satisfied with the U.S. proposal to apportion cargo moving between the two countries in one-third shares to U.S.-flag, Soviet-flag and third-flag vessels, that is not their major concern.

"They want full access to ports, without having to give 14 days' notice" before calling at a port, said Philip J. Loree, executive director of the Federation of American Controlled Shipping.

The Soviets also are interested in gaining access to ocean trade routes between the United States and other foreign countries - known as the cross trades - so they can engage in commerce that produces hard currency for them.

The thinking is that the U.S. insistence on cargo reservation may be a sort of quid pro quo: access to U.S. cross trades for guaranteed business on U.S.-flag bottoms.

That's a recognition that given the current political climate between the two nations, there is "no way" for the United States to say the Soviets can engage in bilateral trade but not the cross trades, said Mr. Loree.

But Mr. Loree called the U.S. cargo reservation plan an idea that ''borders on being ludicrous," and "protectionist lunacy" because it is so "out of sync" with commercial practice.

Shippers, especially shippers of grain and other bulk cargoes also are howling about the cargo reservation provision.