Pace of diplomacy, arms sales pick up in Mideast ... as April deadline nears for Israeli withdrawal from Sinai

Middle East diplomacy is heating up as the April deadline approaches for Israel's withdrawal from the last slice of Egyptian Sinai.

There is also an increase in military nervousness in the area - particularly in Lebanon - as this watershed date draws nearer.

It is a watershed date because, in theory at least, completion of Israeli withdrawal from Sinai brings the parties in the Arab-Israeli dispute to the next major stage in the search for an overall peace settlement. That stage is deciding the future of the Palestinians - and of the West Bank and the Gaza Strip, where most Palestinians under Israeli occupation are concentrated.

These territories (like Sinai) have been under Israeli occupation since the six-day war of 1967. So have the Syrian Golan Heights, whose future Israel sought unilaterally to decide by virtually annexing them last month.

In the overall scenario developing for the next stage of peacemaking, there are elements of hoping or working for the best, but preparing for the worst.

On the working-for-the-best side of the ledger should be listed:

* US Secretary of State Alexander M. Haig's direct involvement by traveling to Egypt and Israel last week. He will be returning to both countries next month. His aim is to smooth the path for the movement of the negotiating process from Sinai to the Palestinians.

* This week's agreement between Egypt and Israel on most of the major outstanding details surrounding Israel's final withdrawal in April. To secure that withdrawal, Egypt has sought to be as accommodating as politically possible in working out these details.

* The current visit to Saudi Arabia of the US State Department's top Middle East man, Assistant Secretary Nicholas A. Veliotes, and the scheduled visit there next month of US Defense Secretary Caspar Weinberger.

Preparing-for-the-worst entries in the ledger depend to some extent on the subjective judgment of those involved, either as initiators or possible targets. They include:

* Continuing deliveries to Israel and the beginning of deliveries to Saudi Arabia and Egypt of some of the more sophisticated new US aircraft. Egypt is also buying aircraft from France.

* The report in the Syrian government controlled newspaper, Tishreen, this week that the Soviet Union has agreed to supply Syria with more arms - without specifying what they would be.

* Syria's efforts - thwarted so far by threat of a US veto - to secure punitive action against Israel through the United Nations Security Council for the Menachem Begin government's virtual annexation of the Golan Heights.

* A revived jitteriness in Lebanon prompted by fears that Israel may use military force against either the Syrians or the Palestinians there before April 25. Over the past week, the Israelis have stepped up their air activity - presumably reconnaissance flights - over Lebanon.

Israel's own Air Force now includes 35 of 40 sophisticated F-15 aircraft ordered from the US. Seventy-five of the slightly less-advanced F-16s have already been delivered.

Saudi Arabia has 62 F-15s on order, of which the first six arrived in the country from the US Jan. 16. Another six are being used in the US to train Saudi pilots.

Egypt has 40 F-16s on order, some of which have already come off the assembly line. Egypt would like more, but in the meantime has ordered 20 Mirage 2000s from France at an estimated cost of $1 billion.

Arab diplomats in Damascus have been quoted as saying Syria wanted enough additional help from the Soviets to achieve a military balance with Israel. But most observers would be surprised if the reported willingness of Moscow to sell more arms to Syria - which gets all its weaponry from the Russians - went as far as matching or even partly matching US aircraft deliveries to Israel.

The Russians have been chronically cautious about over-involvement with their Arab clients. In addition, questions arise about the availability of enough trained Syrian pilots to fly the most modern aircraft - or of Syrian cash to pay for them.

In the crosscurrents of today's paradoxical Arab politics, it is conceivable that Saudi Arabia might advance money to help Syria pay Soviet arms bills - particularly if Syria eventually endorsed the overall Saudi peace plan for the Middle East.

The Saudi plan may well come under discussion during Mr. Veliotes's current visit to Saudi Arabia. Last fall, the Reagan administration gave qualified approval to the plan. The Israelis disapprove of it, because it calls for an independent Palestinian state with east Jerusalem as its capital. The Israeli government argues that this would be the first step toward the dismemberment of Israel.

But to those Americans and Europeans who see something positive in the Saudi plan, it offers an alternative to the current Camp David formula if the latter ends in deadlock over the sensitive issue of the Palestinians.