Libya today announced a merger with Chad, its huge neighbor to the south whose long embattled capital was taken over by government forces three weeks ago with the aid of Libyan tanks and planes.

The unpredictable Libyan leader, Col. Muammar Qaddafi, has sought unity in the last decade with a number of Arab states -- so far in vain -- and is said to be pushing for an association of states under Libyan influence in Africa's Saharan and Sahel regions as part of an Arab Moslem resurgence of which he sees himself as leader.

Unlike previous attempts with Arab states that were never effective -- including one still under discussion with Syria -- the unity declaration with Chad is backed by the presence of Libyan troops and armor. In addition, it comes against the background of Quaddafi's annexation in 1975 of a 60-mile-wide swath of Chadian border territory and reports of promising uranium deposits in the otherwise desolate central African country.

Qaddafi recently told an interviewer that Chad, which is more than twice the size of Texas, is part of Libya's "vital living space."

Diplomats of France, the Western country most concerned by the fate of its former colony, nevertheless, urged caution in interpreting the announcement, recalling previous announcements that were never translated into fact. They conceded, however, that in LIBYA, From A1> this case Libya has the means to enforce its will.

"This," said one official, "is more like the Anschluss [Hitler's merger with Austria] than Qaddafi's platonic unions with Tunisia, Egypt and, now, Syria."

Although Lybia has denied that Libyan forces took part in the conquest of the Chadian capital of Ndjamena on Dec. 15, the joint Chadian-Libyan communique issued today by the official Libyan news agency Jana said Libya would send military experts to "help safeguard security and reinstate peace."

The communique, read over Libya's Radio Tripoli and monitored in Paris, called on all other countries to recognize the government of President Goukouni Oueddei in Chad. Issued at the end of a four-day visit to Tripoli, the Lkibyan capital, by Goukouni, the communique also stipulated that the Chad-Libya frontier will be open "to accomplish the merger of the two brother peoples."

Western intelligence estimates based on satellite reconnaissance of the largely desert country were that about 4,000 Libyan troops took part in the final phase of the 16-year Chadian civil war. Libyan trained Islamic Legion. French and other sources say the Libyan battle plan was worked out by Soviet and East German military technicians, of whom there are large numbers stationed in Libya.

Today's communique warned neighboring Suday "against any aggression against Chad from Sudanese soil." Informed sources say Egypt supplied Goukouni's main rival in the civil war, Defense Minister Hissene Habre, through Sudan.

Jana said that a statement by Sudanese President Jaafar Nimeiri that the war has only just started in Chad "constitutes the start of aggression." After signing a cease-fire, Habre, whose fief is the mountainous country bordering Sudan, swore last week to continue guerrila warfare.

Experts say that the two countries now the most exposed to new pressure from Libya are Chad's eastern and western neighbors, Sudan and Niger. In mid-October, Qaddafi called on the Tuaregs, a leading Saharan desert nomad people, to rebel in northern Niger and Mali and come to Libya as "a base" to conduct their struggle.

"I declare the frontiers of Libya open to the sons of the Tuaregs in Mali and Niger," the Libyan said. "I call on them to revolt, raise their heads and take up arms."

Niger's northern desert contains France's most important source of uranium for its nuclear energy and defense programs. These deposits have helped give rise to the reports that similar deposits could be found in Chad.

French troops have been sent to reinforce the mining region of Arlit in an apparent effort to reassure both the Niger government and the country's large French community.

But France's failure to block Qaddafi's takeover of Ndjamena has obviously shaken faith in French protection in the region. Several French-speaking African leaders have publicly and privately voiced concern over France's hands-off attitude in Chad despite a bellicosely worded French communique just 36 hours before the fall of Ndjamena.

The most populous state in Africa and the immediate region, Nigeria, had been edging toward a tougher stand against Libya and cooperation with France. But, since the fall of Ndjamena, it has reversed itself. At a post-takeover conference in the Nigerian capital of Lagos originally called to discuss Libyan intervention, the only subject publicly talked about was the reconstruction of Chad.

The Nigerian government is said to have been frightened by the implications of the severe rioting by Moslem fundamentalists centered in Kano, northern Nigeria, in the days following the Libyan victory. French officials say there are already small Libyan contingents openly stationed in two Moslem capitals of northern Nigerian states in defiance of the Christian dominated federal government in Lagos.