WHEN former American President Jimmy Carter sits down with central African leaders for a summit opening in Cairo today, he'll confront the continent's most highly charged and seemingly intractable crisis: Hutu-Tutsi enmity.
''The fact that Carter's getting involved in this one is a sign of just how desperate the whole situation has become,'' says one US diplomat based in the region.
More than a year and a half after then-ruling Hutu government troops helped orchestrate a genocide that took half a million mostly Tutsi lives, Rwanda is still caught in the grip of its nightmare.
Sixteen months ago, Tutsi soldiers managed to beat back Rwanda's Hutu-dominated Army. The defeated troops fled across the border, taking with them about one-third of the population, an exiled government, and tens of thousands of armed troops and militia.
Hutu rebel infiltrators along this rugged border with Zaire are becoming more active, further hardening the resolve of the mainly Tutsi leaders who run the government in the capital, Kigali.
Hutu rebels based in eastern Zaire are now better trained and equipped to penetrate deeper into Rwandan territory where they lay land mines and sabotage the infrastructure, military observers here say.
Mainly Tutsi government soldiers recently apprehended infiltrators only 30 miles north of Kigali, and for the first time rebels have begun making incursions into Rwanda through Ugandan territory. International monitors report 50 separate incidents of rebel activity in October.
New rebel tactics are frustrating the government troops. ''The Army can only halfway guarantee security during the day, and not at all at night,'' says a Western diplomat based in Rwanda.
''The [Rwandan] troops seem to be taking it out on civilians. If they can't catch the intruders, they catch the sympathizers in the population instead.''
Human rights monitors in Rwanda say as the rebel infiltration has grown, the government's arrest rates have shot up to some 700 per week. The monitors accuse government troops of killing between five and 10 civilians each day.
Since being ousted, the former Hutu-dominated Army has maintained much of its military structure. Its clandestine cross-border insurgency is sowing fear along Rwanda's frontiers, says Maj.-Gen. Guy Tousignant, who commands United Nations peacekeeping forces in Rwanda.
''The latest infiltrations would drive the best army crazy,'' General Tousignant says. '' If you are very successful at guerrilla warfare, you are making the troops that are responsible for defending your land very, very nervous. And if they are tired, and getting nervous, and they're not successful at catching the perpetrators, then eventually [they] end up overreacting.''
Government soldiers are already overreacting. Outraged by the murder of one of their officers in mid-September, Rwandan troops rampaged through the border village of Kanama and shot villagers dead at point blank range. United Nations officials counted 111 bodies.
Some observers fear such incidents will increase if the approximately 1,800 UN troops remaining in Rwanda leave when their mandate expires on Dec. 8.
Rwandan officials, including Foreign Minister Anastase Gasana, say they don't want the UN mandate renewed and argue that Rwandans are prepared to take responsibility for their own security.
The strength of the rebel threat was dramatized in early November, when government troops launched an early-morning raid on Iwawa Island, just 10 miles off the Rwandan mainland in Lake Kivu, the body of water that separates Rwanda and Zaire.
The troops say they found an estimated 100 rebel fighters and more than 400 civilian trainees on the island with weaponry, including heavy machine guns and antipersonnel and antitank mines.
After a fierce, two-day battle, government soldiers defeated the rebels and took 15 prisoners, among them Hamed Muhawenimana. The teenager says that rebel Hutu officers just across the border in Zaire visit Rwanda refugee camps and lure youngsters into the rebel force with promises of employment.
''A major came to our camp and said he'd get me a job constructing houses on this island,'' Muhawenimana says. ''It wasn't until I was making the voyage here that I finally understood I was going to war.''
Back on the mainland, the mainly Hutu residents of the lush lakeside town of Gisenye quietly admit they fear the Tutsi soldiers. ''It's a climate of mutual suspicion,'' says one resident who did not want to be identified. ''No one can believe you're not with the other side.''
The mistrust has run even through the higher reaches of the government. In August, Rwandan officials ousted four moderate ministers, ostensibly on the grounds that they were incompetent.
Privately, many leaders say some of the ministers were not loyal to the government.
Among those ousted was Interior Minister Seth Sendashonga. He says popular confidence in the government is eroding, especially in rural areas, where the villagers are mainly Hutu.
''The government had a lot of support from the population because it had saved it from horrors,'' Sendashonga says. ''Now residents have witnessed a lot of injustice, mass arrests of people who are known to be innocent by the population. It is not surprising that part of the population may be turning their ears to the other side.''
Another of the ousted ministers says he and his colleagues lost their government positions because they dared to challenge the rule of powerful vice president and Armed Forces chief Gen. Paul Kagame and because they lobbied for added security to ensure the safety of returning refugees.
''No minister can say the contrary of what Kagame says,'' says the ex- minister, who doesn't want to be identified. ''That's the lesson the Cabinet has learned.''
Sendashonga says most Rwandans currently don't support the rebels, because the fighters are linked to those who perpetrated last year' s genocide. But he says the rebel threat could gain support if the government does not stop Army abuses.
''Any threat can only be meaningful to the extent that it has support from within,'' Sendashonga says.
''And I'm afraid if the government does not change course radically to reassure all the population, some few elements may be tempted to go collaborate with the infiltrators,'' he says.
But with the Rwandan Army facing a growing threat inside and outside its borders, it seems unlikely to become more tolerant.
''On the other side of the border there are 30,000 men under arms, a complete government, and one-third of the population. Here there are 50,000 men under arms, and a minority regime,'' a Kigali-based diplomat says. ''The situation is potentially explosive.''