American envoy Philip Habib arrived here on a trouble-shooting mission today after driving within a stone's throw of some of the Syrian ground-to-air missiles that have increased the anxiety in the Middle East.

Habib's arrival came as the situation was further strained by a news agency report of provocative Syrian troop movements. An Associated Press dispatch quoted Lebanese government sources as saying that two Syrian armored brigades -- including tanks, armored cars and 4,000 troops -- had violated the boundary set up to keep them from deployment in southern Lebanon.

The report was greeted with skepticism by military specialists here, and it had no independent verification or identified sources. It was impossible to determine where the report originated or why.

[The Israeli Army command reported Thursday night that it had "no knowledge" of the reported Syrian troop movements, Washington Post correspondent William Clairborne reported from Jerusalem.]

[An official of the United Nations Interim Force in Lebanon (UNIFIL) also said that no Syrian concentrations had been observed in the alleged areas of southern Lebanon.]

[Timur Goksel, a UNIFIL spokesman, said in a telephone interview with Claiborne that a U.N. observation post is located immediately adjacent to the village of Tibnit, where the Syrian troops were reported to have taken positions. Tibnit is five miles from the northernmost Israeli village of Metullah.]

["There has been no Syrian brigade -- forget a brigade, a company of light infantry could not move there on those deserted roads without being seen by our observation posts," he said. "It is a very exposed area. I know that area quite well. We have adequate observation there, and there has been absolutely no movement of Syrian forces into that area."]

Nonetheless, official Israeli radio broadcast the AP dispatch on its Arabic, English and Hebrew news programs, and the Israeli-backed rightist Christian militia spokesman said Syrian troops were south of the boundary, but he declined to identify where they were stationed.

Palestinian sources voiced "great concern" about the reports which they described as an "obvious provocation."

The Syrian troop movement, if it had been confirmed, could have triggered an Israeli intervention.

Under the 1976 tacit agreement governing the Syrian military presence in Lebanon, there is no limit on the number of troops deployed. But the Israelis insisted that Syria could not deploy ground-to-air missiles or send its troops south of a line running roughly from the Zahrani River mouth on the Mediterranean due east to the Syrian border.

Syria deployed the missiles in Lebanon last week after Israel intervened in the latest round of Lebanese fighting to shoot down two Syrian helicopters. Military sources reported today that Syria added 7,000 fresh troops to its Lebanon contingent.

Since Beirut Airport has been closed for 16 days because of the fighting here, Habib, a former undersecretary of state, landed in Damascus and was driven here where he will begin trying to defuse the current crisis. Habib will meet the Lebanese President Elias Sarkis and other officials and Christian leaders Friday and then go to Damascus to meet Syrian President Hafez Assad. He is scheduled to travel to Israel Sunday to see Prime Minister Mencachem Begin.

While the American initiative was just beginning, Assad and Foreign Minister Abdul Halim Khaddam were conferring with Georgy Korniyenko, the Soviet first deputy foreign minister who arrived in Damascus from Moscow yesterday.

Syrian official sources said the two-hour talks dealt with "Israeli challenges in Lebanon, encouraged by the American attitude, aligned with Israel."