WASHINGTON--President Reagan ended a 10-day California vacation yesterday, returning to work on a renewed campaign against international terrorism in the wake of another wave of attacks aimed at Americans.

While stopping short of blaming Libyan leader Moammar Khadafy for two attacks last week in which Americans were killed, an administration official accused Khadafy of sponsoring "a master plan to cause terrorist incidents worldwide."

Reagan, who spent a working vacation at his mountain-top ranch near Santa Barbara doing chores and horseback riding, was briefed on the explosions on a TWA airliner approaching Athens and in a West Berlin discotheque popular among U.S. soldiers.

As he boarded Air Force One at Point Mugu Naval Air Station, Reagan was asked by a reporter whether he believed Khadafy was behind the West Berlin bombing. He said, "No comment."

Asked whether he planned to "hit Libya," Reagan again answered: "No comment."

The Berlin bombing prompted an administration official, who spoke on the condition he not be identified by name, to accuse Khadafy of masterminding a plan for world terrorism.

The official said the United States was pursuing diplomatic initiatives to increase cooperation with European allies to combat terrorism.

Earlier in the week, the White House responded cautiously to placing blame on Libya as the possible agent behind the bomb that killed four Americans on the TWA flight bound from Rome to Athens on Wednesday.

In Thursday's news briefing, however, White House spokesman Larry Speakes stressed the Reagan Administration was not ruling out Khadafy as a possible suspect.

On Friday, a bomb exploded in a West Berlin nightclub, killing a U.S. Army sergeant and his companion and injuring nearly 200 people, some of them American military personnel.

"There is obviously a master plan from Khadafy to cause terrorist incidents worldwide, and particularly they've targeted Americans," the official said.

More than 30 U.S. facilities have been targeted for potential terrorist attack by agents of the Khadafy government in recent months, the official said. He added that an unspecified number of diplomats have been followed by Khadafy agents.