Lebanon's U.N. mediated cease-fire was broken today when Israeli and Lebanese Christian militia gunners shelled the port city of Tyre, Beirut's state-run radio said.

But in Tel Aviv, Israeli military sources said the reports of new heavy fighting were exaggerated, and they denied that Israeli forces were involved.

Beirut radio also reported an exchange of rocket fire between rightists in Qleyaa and Palestinians and their leftist allies in Arnoun, but indicated the clash was a minor one.

The new fighting came as an envoy of Lebanese President Elias Sarkis, Rene Moawad, flew to Syria to begin a tour to urge Arab leaders to call a summit on the continued Lebanese fighting.

In Damascus, Syrian President Hafez Assad met today with West German Foreign Minister Hans Dietrich Genscher for talks aimed at narrowing differences over Middle East policy.

At a state banquet last night, Genscher defended U.S. Middle East policy after Syrian Foreign Minister Halim Khaddam charged that the United States was pursuing a "colonialist" policy.

Genscher said: "Whoever has followed political events in Europe cannot overlook the fact that, without their alliance with the United States, the states of the European Community now would be subjected to foreign domination . . ."

Genscher also defended Egyptian President Anwar Sadat's agreement to a peace treaty with Israel. "The hesitation of some of those involved to

In other developments: take the first step should not prevent all necessary steps from being taken," Genscher said. Khaddam, in an earlier toast, called Sadat's action "treason against the Arab nation."

Israeli sources said a Cabinet meeting will be held soon to debate Israel's policy in southern Lebanon. The matter was raised yesterday by Foreign Minister Moshe Dayan, who called on the government to weigh military gains in southern Lebanon against the damage to Israel's reputation abroad, sources said.

Syrian officials asked Saudi Arabia to pressure Israel to stop shelling Lebanon, while Lebanese Foreign Minister Fuad Butros called on the United States to pressure Israel too.

The southern Lebanese port of Sidon was paralyzed by a general strike called to protest clashes between two Palestinian commando organizations. Two persons were reported killed and 10 wounded in clashes between Fatah and the Democratic Front for the Liberation of Palestine.

Delegations of elders from a number of Shiite Moslem villages near the Israeli border demonstrated in Kfar Kila, Lebanon, today to protest recent shellings in the area by Palestinians and other leftist forces. The demonstrators said that six persons had been killed by shelling in the villages during the weekend.

Syrian President Assad said in a magazine interview that efforts toward unification between Syria and Iraq had failed, at least for the time being. "Perhaps it is necessary and advisable to allow for some time to pass before our brothers in Iraq are willing to rejoin the common path," Assad was quoted as saying. "This depends solely on the leadership in Iraq . . ."