Israeli Prime Minister Menachem Begin said today that there is a limit to how long Israel can wait for success of U.S. diplomatic efforts to ease tensions over Syrian missiles in Lebanon, because of a continued Syrian military buildup there.

Begin did not say what the time limit is nor did he indicate that he has given up on the shuttle diplomacy of special U.S. envoy Philip C. Habib, who is expected to return to the region early next week.

But Begin emphasized, in comments to reporters at his office today, that diplomatic attempts to end the Syrian-Israeli confrontation had yielded "no practical results."

"Of course there is a limit. I won't say what that limit is, but it is impossible to carry talks back and forth without any practical result," Begin said in response to a question.

It was the first time that Begin said Israel considered Habib to be under a time limit. In past statements on Habib's mission, Begin had said there was no time limit.

Begin said today that the Syrian surface-to-air missile batteries are still in central Lebanon, that Syria has offered no commitment not to fire the missiles at Israeli aircraft and that Syrian forces are still attacking Israeli-supported Lebanese Christian forces in the Mount Lebanon range and besieging the Christian city of Zahle.

Begin stressed that Israel does not seek war with Syria and he reiterated his pledge that Israeli armed forces will not attack Syria unless attacked first. But implicit in his remarks was the threat that ultimately the Israeli Air Force would strike against the missiles if they are not removed.

His oblique warning, when viewed against the backdrop of the quickening pace of his campaign for reelection in the June 30 national election, closely followed Begin's recent pattern of mixing strident anti-Syrian statements as prime minister with demonstrable military restraint in his role as Israel's defense minister.

This tactic -- at least for as long as the situation remains in a state of suspended animation -- appears to have guaranteed Begin public exposure dominance over his rival, opposition Labor Party leader Shimon Peres.It has also made it difficult for Peres to get the electorate's attention by attacking Begin on the issues on which he is most vulnerable, such as 130 percent annual inflation and the deterioration of Israel's economy.

Begin today also tacitly confirmed reports that he turned down requests by the Reagan administration to suspend Israeli air and ground strikes against Palestinian guerrilla positions in Lebanon until Habib completes his diplomatic mission.

Israeli sources said that the U.S. ambassador to Israel, Samuel Lewis, in two meetings with Begin last week, had transmitted a Habib request for a moratorium on Israeli air strikes against the guerrillas, arguing that they would only harden Syria's position and make it more difficult for Saudi Arabia in its efforts to mediate an agreement among Syria, Lebanon and, indirectly, Israel.

Asked today whether the United States had requested such a moratorium, Begin said Israel will "continue to strike the terrorists in their bases" in Lebanon. He said such strikes are the only way to defend Israeli civilians from cross-border terrorist raids.

Begin said today that the Syrian army has been conducting maneuvers and has vastly holstered its forces in Lebanon and close to the Lebanese border.

In a background paper issued to foreign journalists and attributed to military intelligence analysts, the Israeli military warned that "wideranging preparations are being completed in all branches of the Syrian armed forces. These preparations are not only defensive in nature but also enable the Syrians to initiate an offensive against Israel -- both on the Lebanese and Golan Heights theaters of action."

The military preparedness, the report said, includes reinforcing the missile batteries, building trenches and earthwork fortifications, large-scale movement of troops, deployment of a tank brigade along the Lebanese border, movement of an infantry battalion to Lake Karun, seven miles north of the Israeli border, and the callup of thousands of reservists.

Deputy Defense Minister Mordechai Zippori noted today, however, that while Syria seems to be completing preparations for war, it was careful not to intervene with its air force when Israeli warplanes on Thursday bombed Libyan-manned missiles south of Beirut in an operation that reportedly lasted 90 minutes.