Palestinian gunners in southern Lebanon fired rocket salvos at Israeli settlements today in the third consecutive day of cross-border warfare between Israelis and guerrilla forces.

Artillery shells fired from Israel and Lebanese Christian militia forces hit two key targets in southern Lebanon -- bombed by Israeli jets yesterday -- Palestinian-held Beaufort Castle and the market town of Nabatiya, journalists in the area reproted.

The attack on Israel by Soviet-made rockets caused some damage but no casualties and was an apparent reprisal for a series of Israeli ground and air attacks on Palestinian guerrilla bases in southern Lebanon since early Tuesday, the biggest in 2 1/2 years.

The Israeli government, meanwhile, rejected a U.N. Security Council resolution declaring "null and void" Israel's new Jerusalem law and expressed regret that the United States did not veto the resolution rather than abstain.

The resolution, adopted on a 14-0 vote yesterday, with the United States the sole country abstaining, also called on 13 nations with embassies in Jerusalem to withdraw them.

Following the vote, Chile became the fourth South American nation to announce that it would move its mission from Jerusalem to Tel Aviv. Venzuela, Ecuador and Uraguay had already announced they would transfer their embassies.

Israeli Foreign Minister Yitzhak Shamir met U.S. Ambassador Samuel Lewis to express "Israel's regret" that the United States had not vetoed the resolution. He said Secretary of State Edmund Muskie's speech to the Security Council "contained convincing reasons for the United States to oppose the resolution and not just to abstain."

Sources close to Israeli Prime Minister Menachem Begin said it was possible he would go ahead and move his office from predominantly Jewish West Jerusalem to the annexed Arab sector soon in defiance of the Security Council vote.

Meanwhile, Egyptian President Anwar Sadat told the Israeli newspaper Maariv that he and President Carter had "serious differences of opinion" over how to resume the stalled Palestinian autonomy talks.

Sadat, who has proposed a new summit with Begin and Carter after the U.S. elections, was quoted in an interview with the newspaper as saying, "The American president wants the talks to continue, and he's trying to find a possible route to renew them. I also want the talks to continue, but how?"

[In Washington, a spokesman for the White House said it had "no comment" on Sadat's comments regarding his differences with President Carter over renewing the talks.]

Egypt interrupted the talks following the passage of the Israeli law July 30 formally proclaiming all Jerusalem the eternal and undivided capital of the Jewish state.

In another development, Israel announced today it has arrested a 24-year-old Arab in the January slaying of an Israeli Army private in the West Bank city of Hebron. His name was not released.

The murder of Yehoshua Sloma, 23, a student at a Jewish seminary who also served in the Army, was the first in a series of violent acts this year in the Israeli-occupied West Bank.