The White House yesterday condemned both the violent Palestinian demonstrations in Israeli-occupied territories and Israel's "harsh security measures" in dealing with them while the United States abstained as the U.N. Security Council approved a resolution deploring Israeli policies in the territories.

In a photo-taking session, President Reagan called the situation "regrettable." When a reporter noted that Israeli leaders had said they were not concerned how international opinion reacted to the situation, Reagan said: "Well, they may not be concerned, but maybe the world is concerned."

Both the White House criticism and the abstention at the United Nations came despite the Reagan administration's staunch support of Israel over the years.

"For the United States to abstain from a resolution criticizing Israel is tantamount to supporting the resolution," one Arab League official said, speaking on condition of anonymity. "For the United States to abstain is a victory for us."

U.S. deputy permanent U.N. representative Herbert Okun said the United States abstained because the resolution "ignores the fact that Israeli lives are also at risk and that Israeli security forces have been faced by provocations and, in some cases, by life-threatening situations.

"To the best of my government's knowledge, these demonstrations were spontaneous expressions of frustration, and not externally sponsored," Okun said.

"This latest eruption of violent confrontation is a forceful reminder of the unresolved political status of the West Bank and Gaza as well as of the deep frustration of the Palestinian people whose daily lives are so profoundly affected."

The carefully drafted White House statement said both sides shared responsibility for the worst outbreak of violence since Israel occupied the West Bank and Gaza Strip in 1967 and urged Israelis and Palestinians alike to "step back from confrontation" to avoid further loss of life.

"Demonstrations and riots on one side and harsh security measures and excessive use of live ammunition on the other cannot substitute for a genuine dialogue," White House spokesman Marlin Fitzwater said.

The statement said the Israeli government's occupation of the two Arab territories was "exacting a toll" on the 1.5 million Palestinian inhabitants "and on Israel as well."

"The effects of occupation are not felt in the territories alone. They also damage the self-respect and world opinion of the Israeli people," Fitzwater added.

Fitzwater said Reagan had personally been monitoring the situation "very carefully," discussed it with his national security adviser, Lt. Gen. Colin L. Powell, and "been involved in the communications" the administration has sent to Israel and some Arab governments regarding the unrest.

"We have stressed that we would like to see an end to the violence," Fitzwater said. "We have asked for restraint in the use of live ammunition and have made our views known in public as well as diplomatic channels."

Fitzwater suggested that the White House issued the blunt statement now, after two weeks of unrest and at least 21 Palestinian deaths, because the situation had "not resolved itself, tensions have not lessened. It becomes more serious all the time."

Despite the criticism of Israeli tactics, there was no indication from Fitzwater or State Department spokeswoman Phyllis Oakley that the administration intends to step up pressure on Israel to change its harsh methods.

U.S. officials said Monday that U.S. Ambassador Thomas Pickering met with Israeli Prime Minister Yitzhak Shamir to discuss the situation.

The State Department called upon Israel and the Palestinians to deal with their "underlying problems and frustrations" through negotiations. "It is time for all sides to look beyond narrow definitions of the causes of this violence and to commit themselves squarely to a process of negotiations," Oakley said.

She said the United States remained ready to help but gave no indication the administration was planning any new initiative to revive the Middle East peace talks.

The abstention by the United States, which, as one of five permanent members of the U.N. Security Council can prevent passage of resolutions by its veto, let the resolution stand on a vote of 14 to 0.

Since the emergency Security Council session that began Dec. 11, several drafts of the resolution had been proposed and revised. An agreement among the 15 member nations appeared nearly certain on Monday, but the Palestine Liberation Organization insisted on a change in wording from "strongly deplore" to "condemn" and demanded other revisions.

The United States balked at the demands of the Palestinians, according to diplomats, but went along with strongly deploring Israel's action and abstaining from voting.