Strange Bedfellows in the Mideast. Palestinian and Shiite radicals in Lebanon are joining forces against the US and PLO. TERRORISM: UNHOLY ALLIANCE

UNITED STATES and European terrorism specialists are keeping a close eye on signs of new cooperation among Lebanon-based terrorist groups. ``It's a case of cooperating against common enemies,'' says a US specialist on the region. ``Only a few months ago, Hizbullah [the pro-Iranian Shiite grouping in Lebanon] and the PLO [Palestine Liberation Organization] were passing arms and intelligence back and forth. Today, the PLO has a truce with Amal [Hizbullah's bitter Lebanese rival] and is talking to the US. Hizbullah remains committed to fighting Israel. It thus finds a lot of common ground with the Palestinians who oppose the PLO.''

``This probably isn't a permanent alliance,'' a US official adds. ``But as long as this alignment lasts, it's very dangerous for all of us perceived as their common enemies.''

Longtime Lebanon-watchers say alliances regularly shift, a feature of the chaos caused by Lebanon's 13-year civil war. ``This underlines that we just can't shunt Lebanon aside. It spills out into the whole region and beyond,'' a ranking US diplomat says.

From a counterterrorism perspective, the prospect of increased cooperation among the likes of Hizbullah, the Abu Nidal Organization (ANO), and other radical groups is daunting.

``The ANO, the terrorist wings of Hizbullah, and groups like the Popular Front for the Liberation of Palestine - General Command are each run on the basis of small, tightly organized cells, which operate independently from each other,'' says a well-placed US counterterrorism specialist. ``They all have international reach. If we get a serious networking between these groups, it will be extremely hard to track and to crack.''

A specific focus of attention is the so-called ``Soldiers of Truth,'' or Jund al-Haq. This Islamic-sounding name may be the cover for increased cooperation between ANO and radical Shiites, Western specialists say.

The Soldiers of Truth first appeared in December, when they claimed responsibility for the May 1988 kidnapping of a Belgian doctor. The group subsequently claimed an arson attack against a Jewish center in Copenhagen, and attacks on Saudi diplomats in Thailand and Pakistan.

Another Saudi diplomat was attacked in Turkey last October. That murder was claimed by another Islamic-sounding group, the Islamic Jihad in the Hijaz, as retribution for the execution by the government of Saudi Arabia of four pro-Iranian Saudis for terrorism. The Islamic Jihad in the Hijaz also claimed the January killing of the Saudi diplomat in Thailand.

Press reports from Beirut suggest the two groups are fronts set up by Iranian radicals and their Lebanese allies opposed to Iran's moderation and - specifically - to improving relations with Saudi Arabia.

US and European specialists suspect these are, in part, new cover names for actions by existing Hizbullah and/or Iranian terrorist networks. But they say the Belgian doctor, Jan Cools, was kidnapped by ANO and reportedly remains in ANO custody at present, despite the claim issued in the name of the Soldiers of Truth last December.

ANO is believed to have held a Swiss member of the International Committee of the Red Cross (ICRC) last November and December. The initial demand for his release was the liberation of a Hizbullah member held for hijacking in Switzerland.

The kidnappers, however, eventually settled for the promise of ransom, say well-informed Western officials. When no ransom was paid, the ANO is reported to have issued clandestine death threats against ICRC workers. This led the ICRC to pull out of Lebanon in late December.

Abu Nidal is believed to have terrorist cells in each of the countries where the Soldiers of Truth claimed to act, as well as in Turkey. Last year, the ANO is thought to have carried out an attempted attack on a Saudi consulate in Bombay. This closely followed reports that ANO was offering its services to Iran.

A well-informed French official says ANO has been building bridges with Islamic fundamentalists in south Lebanon for some time, and that it has been recruiting them for operations. The more formal cooperation which may be emerging now, he says, was thus a fairly easy step.