U.S. and PLO officials began a second round of official talks Wednesday with the American side ready to deliver some "tough words" about terrorism and cross-border raids on Israel.

A three-man U.S. delegation led by Ambassador Robert Pelletreau sat down with three members of the Palestine Liberation Organization shortly after noon in a government-owned guest house in the ancient port of Carthage, 30 miles northeast of Tunis.More than four hours later, the talks continued behind closed doors - far longer than political observers had expected. The first round of talks lasted only 90 minutes.

At the start of the talks, Yasser Abed Rabbo, who led the PLO delegation, addressed Pelletreau in Arabic, saying, "Just one year ago today the United States decided to close the PLO office in Washington. And now here we are . . . one year later to begin an official dialogue."

An American Embassy source said shortly before the talks began that the United States would take a tough stand on terrorism but stop short of demanding an end to the Palestinian uprising in the Israeli-occupied territories.

The embassy official, who requested anonymity, said the U.S. side also wanted to find out if the PLO would agree to direct talks with Israel before the holding of an international conference on the Middle East.

"The U.S. side will have some tough words to say about our concern on the question of terrorism and our expectation that (PLO) Chairman (Yasser) Arafat will live up to his commitments made in Geneva," the source said.

The Reagan administration decided to open the dialogue with the PLO last year after Arafat, in a December speech before the United Nations in Geneva, renounced terrorism and implicitly recognized Israel's right to exist.

The first U.S.-PLO session took place Dec. 16 despite strong objections from Israel. It was the first time the two sides met since the formation of the PLO in 1964.

Wednesday's second session marked the first formal meeting under the Bush administration.

Pelletreau, dressed in a gray suit and a red-and-black tie, sat at one rectangular table alongside embassy political officer Edmund Hull and deputy chief of mission Gordon Brown.

Rabbo, a PLO Executive Committee member and official of the Democratic Front for the Liberation of Palestine, sat at another table facing the U.S. delegation. The pro-Soviet DFLP is one of the most radical factions within the PLO.

Rabbo was joined by Hakam Balaawi, the PLO representative to Tunisia, and Abou Jaffar, assistant to the PLO's director of political affairs.

The question of terrorism is a key item on the agenda. The United States is seeking to discourage any Palestinian attacks on Israel, regardless of whether they can be defined as terrorism and even if they are undertaken by splinter groups technically outside Arafat's control.

"It's a question of overall atmospherics and what contributes to an atmosphere of peace or not," the embassy source said. "We take the position that any (attack) action is not one that is conducive to creating an atmosphere for negotiations."

But he said the U.S. side would not demand an end to the Palestinian uprising in the Israeli occupied territories.