Syria is ready to consider the idea of a United Nations force to replace American and other Western troops in Beirut. And it feels assured of Soviet backing on the issue.
The Syrians, who are feeling vindicated by the woes of both the United States Marines and the US-backed government of Lebanese President Amin Gemayel, are insistent that the Americans change their ''policy in Lebanon and the Mideast'' as part of any such arrangement, acting Information Minister Farouq Sharaa implied at a news conference here Sunday.
A series of remarks, public and private, by various officials here suggest Damascus is thinking in terms of the following specific conditions:
* The Marines must leave Lebanon altogether, not just redeploy on US naval vessels offshore. The naval force, at a minimum, must stop shelling Syrian-controlled areas of Lebanon, and probably would have to move out completely.
* President Gemayel - although the Syrians seem less insistent than Lebanese opposition leaders that he must resign - would have to scrap his US-mediated May 1983 peace agreement with Israel.
* The Israelis, too, must pull out of Lebanon.
* Any UN force must have a specific mandate limited in time and confined to Beirut. The force should exclude members of the present multinational force - the US, Italy, France, and Britain.
Some Western analysts here feel these conditions might be toned down somewhat during any serious new dialogue between Damascus and Washington.
But on that general front, Mr. Sharaa gave a terse verdict of, ''no progress.'' He argued that the Americans have yet to show any willingness to amend what he portrayed as a totally pro-Israeli, anti-Syrian, approach to Mideast issues.