As American special envoy Philip C. Habib picks up the threads of his Lebanon peace mission, Israel is worried by two overriding issues: * The growing military strength of the Palestine Liberation Organization. Israeli sources say that the Soviet Union quietly has been upgrading the PLO's military arm in recent months from a guerrilla-terrorist group to that of a conventional fighting force.
* The still-unresolved crisis over Syrian missile emplacements in southern Lebanon. Israel called off its planned air strike against the missiles earlier this year only because of the urgent intercession of the US government which sent Mr. Habib to the region to find a diplomatic solution.
But Israeli officials are not optimistic about Mr. Habib's chances of untangling the complicated situation. His intensive efforts last summer, they note, achieved a cease-fire but no basic changes in the region.
Since the July 24 cease-fire, Israeli sources say, the Palestinians have received 60 T-34 tanks, 200 artillery pieces, 80 armored personnel carriers, and ground-to-air SAM rockets.
These weapons supplement previously supplied equipment, including the multibarrel Katyushas and long-range artillery that the PLO used to great effect during its 10-day skirmish with Israel in July.
According to Israeli analysts, the Soviet steps are a signal to the West that there will be no Middle East settlement unless its client, the PLO, is brought into the negotiations. The analysts note that Moscow's diplomatic recognition of the PLO - the first by a non-Arab state - came two weeks after the assassination of Egypt's Anwar Sadat.
Israeli military sources say that at least 1,000 Palestinians have received military training since 1974 in the Soviet Union, Cuba, and other East bloc countries. Russian training material was captured by Israeli forces when they overran PLO camps during their takeover of southern Lebanon three years ago. Graduation certificates issued to PLO trainees were also found.
Israeli military sources say the training ranges from command and staff courses to pilot training to courses in explosives. Citing Western sources, the Israeli Army spokesman declared recently that training centers for Palestinians are located in Moscow suburbs, at Patrice Lumumba University, and in Baku, Tashkent, Odessa, and Simferopol.
Israel is keeping close track of the PLO's rearming and reorganization in south Lebanon. Israeli chief of staff Rafael Eitan has publicly expressed doubt that the cease-fire will long endure, attributing possible infringement to the PLO.
If fighting resumes along the Lebanese border, it will most likely be of far greater intensity than that which shook the region in July.