The Reagan administration displayed heightened concern here tonight about military developments in the Middle East, speaking for the first time of "aggressive behavior" by Syria and describing the situation as "dangerous."

The U.S. statements came at a press briefing by senior officials at the beginning of the seven-nation summit here, attended by President Reagan, Secretary of State George P. Shultz and Treasury Secretary Donald T. Regan.

The State Department had issued a public statement late Friday criticizing a "Syrian buildup of forces in Lebanon and along the Syrian-Lebanese border" and calling for the "utmost restraints" to reduce the risk of conflict.

Tonight, a senior State Department official went further, saying:

"The additional Soviet weaponry, the Soviets manning that weaponry, the aggressive behavior of the Syrians, their association with PLO Palestine Liberation Organization guerrilla forces, with Iranian terrorist groups that are there, all provide a situation that is dangerous."

The senior official, who cannot be named under ground rules of the briefing, said that both the United States and Israel are watching developments carefully. He added that Israel has been displaying "restraint" in the face of "quite a large Syrian buildup."

State Department sources said that the heightened concern flowed not so much from recent Syrian military maneuvers as from a puzzling and continual increase in Syrian forces sent into Lebanon and along the Syrian-Lebanese border.

One official also said that Syria is "making noises" along the border which are unwelcome. He gave no details of this activity.

The expressions of U.S. concern followed messages from Israeli officials earlier this week and a visit by acting Israeli ambassador Benjamin Netanyahu to Shultz on Friday.

There was no report of U.S. military movements into the Middle East or of increases in U.S. military preparedness. But there were some indications that U.S. forces previously scheduled to leave the Middle East area will stay on because of the increased tension.