Defense Secretary Caspar W. Weinberger, returning from talks with French and Saudi Arabian defense ministers in Paris, yesterday said the trip had increased his optimism about achieving a settlement in Lebanon.

Weinberger declined to discuss the substance of his conversations with Prince Sultan, the Saudi defense minister, but he acknowledged that Lebanon was on the agenda. "I just feel optimistic as a result of these meetings," Weinberger said as he stepped off a helicopter at the Pentagon.

Other Pentagon officials played down recent reports of Syrian and Palestine Liberation Organization troop buildups in the Lebanese Bekaa Valley.

But Sen. Henry M. Jackson (D-Wash.) said the Syrian movements had created a "very, very dangerous" situation with "the potential for explosion."

"The buildup of troops in the Bekaa Valley has not been minor, it's been major," Jackson said during a breakfast meeting with reporters. "I want to emphasize that anything could happen there anytime."

Administration officials said earlier this week that a convoy of 600 to 800 Syrian and PLO fighters had entered Lebanon. They cautioned that the movement might represent a normal rotation of troops.

Jackson, a member of the Senate Intelligence Committee, declined to reveal the source of his information. But he said the number of Syrians in Lebanon was "well in excess" of the 35,000 that had been reported. He said the situation has worsened in the last 24 hours, with the Syrians deployed in increasingly offensive formations.

Pentagon officials said that, rather than preparing for war, the Syrians may be trying to pressure the Lebanese government to reject an agreement, recently negotiated by Secretary of State George P. Shultz, that would lead to withdrawal of Israeli and Syrian troops from Lebanon.