Israel's new ambassador to the United States warned yesterday that Palestine Liberation Organization forces in southern Lebanon, bolstered by a buildup of heavy weaponry, seem increasingly likely to take "some provocative action" that will force Israel to retaliate militarily.

"You might almost say it's a matter of time," Moshe Arens told reporters in assessing the likelihood that the fragile cease-fire in effect along the Israeli-Lebanese border since last summer might be shattered soon.

However, Arens' remarks left unclear whether he was hinting that Israel is preparing a new strike into Lebanon or is attempting to put pressure on the United States to work toward removal of the PLO's weapons from areas where they can be fired across the border into Israel.

President Carter's special Mideast envoy, Philip C. Habib, who negotiated the cease-fire, is just beginning a new mission aimed at defusing the new tensions involving Israel, the PLO, anti-Palestinian Lebanese Christian militias and Syrian forces inside Lebanon.

The Habib mission has some urgency about it, because the United States fears that Israeli Prime Minister Menachem Begin's government may be preparing to repeat its 1978 invasion of Lebanon. For the past several weeks the Reagan administration has been putting heavy pressure on Israel to avoid such a course.

Earlier this month Begin was understood to have overruled at the last minute a plan by his defense minister, Ariel Sharon, and the Israeli chief of staff, Lt. Gen. Rafael Eitan, for a strike against PLO strongholds in southern Lebanon that could have escalated into a wider war.

However, Secretary of State Alexander M. Haig Jr. has acknowledged that the PLO forces in the region have been receiving large shipments of Soviet missiles and artillery and that Israel's continued "forebearance" depends on the PLO's behavior.

Other U. S. officials, while confirming the arms buildup, have said privately that the PLO so far has refrained from deploying its troops in significant numbers near the Israeli border.

Arens was even more explicit yesterday. He said northern Israel is "under PLO guns" that are capable of inflicting heavy casualties within the Jewish state, charged that the PLO is likely to use these weapons in the near future and implied that the only hope of averting a major military clash depends on Habib and the removal of the PLO's heavy arms from southern Lebanon.

Arens, who has been in his job here less than two weeks, also elaborated "unofficially" on Sharon's statement this week that Israel will not permit Jordan to buy sophisticated new weaponry such as mobile missiles and advanced jet fighters from the United States.

He said Sharon was reflecting Israeli concern that the U.S. policy of trying to make friends by increasing its arms sales to Arab countries is forcing Israel "into a corner where it would have no choice but to take some kind of preemptive action" against an Arab nation whose arms buildup threatened to tip the Mideast military balance against the Jewish state.

Arens, the former chairman of the defense and foreign policy committee of the Israeli parliament, contended that the administration's arms sales policy is reducing Israel's "qualitative edge" against the combined Arab world forces to the point where it no longer can "absorb a first strike by the enemy and still win the war."

That, he said, is why there is increasing talk in Israel of the possible need for preemptive actions.

The ambassador, who is regarded in Israel as an outspoken hawk on military questions, insisted that his government has "differences of opinion, but not conflicts of interest" with the United States, and he added that the two countries are in broad agreement on most aspects of Middle East policy.