NDJAMENA, Chad — American military officials say that two of the world’s most feared terrorist groups — the Islamic State and Boko Haram — have begun to collaborate more closely, raising alarm that they are working together to attack American allies in North and Central Africa.

On Wednesday, Brig. Gen. Donald C. Bolduc, the commander of the United States military’s Special Operations in Africa, cited a weapons convoy believed to be from Islamic State fighters in Libya that was headed for the Lake Chad region, an area devastated by Boko Haram.

Military officials described the convoy as one of the first concrete examples of a direct link between the two extremist groups since Boko Haram pledged allegiance to the Islamic State last year. The shipment, seized near the Chadian border with Libya on April 7, was carrying small-caliber weapons, machine guns and rifles, officials said.

The disclosure came during a tense series of meetings here in the capital of Chad between Samantha Power, the United States ambassador to the United Nations, and top officials like President Idriss Déby, who is expected to announce soon that he won recent elections and will begin his fifth term in office. He seized power in a coup and has governed Chad with a firm hand for 26 years.

Mr. Déby said Ms. Power had pressed him during their meetings about reports that dozens of Chadian soldiers who had voted for opposition candidates had suddenly disappeared.

“No one has disappeared,” Mr. Déby told reporters, standing next to Ms. Power after their meeting and responding to a query from a reporter. “They will be presented to the world on television.”

The exchange highlighted a central quandary of President Obama’s efforts to rein in the spread of Islamic extremism in Africa. In recent years, the continent has increasingly become a battleground in the West’s war against militant Islam. Administration officials insist that the increased influence of groups like Boko Haram, and now the Islamic State, has some of its roots in the economic disparities and human suffering often brought on by authoritarian governments in which strongmen cling to power.

But at the same time, the United States is supporting such governments as they battle Boko Haram and other extremist groups. In the Lake Chad area, which includes countries like Nigeria, Niger, Chad and Cameroon, American Special Operations forces are training and advising African militaries in the fight against Boko Haram, Al Qaeda in the Islamic Maghreb and now the Islamic State.

“The Lake Chad basin is ground zero” in the fight against militant Islam in Africa, General Bolduc said.

He said that beyond Boko Haram’s pledge of allegiance to the Islamic State last year, the two groups were sharing “tactics, techniques and procedures.” His comments amplify concerns expressed recently by other American officials that the Islamic State’s branch in Libya is deepening its reach across a wide area of Africa, attracting recruits from countries as far away as Senegal that had been largely immune to the jihadist propaganda.

Mr. Déby, the Chadian president, called the Islamic State a “monster” that recruited Chadians, just as it had recruits from Europe, the Middle East and other parts of Africa. At times, he sounded like he was speaking from an Obama administration set of talking points.

“We cannot eradicate terrorism by military action alone,” he said. “We need to look to our youths, and to work not to be a platform” for terrorist groups.

Mr. Déby led a rebellion to take power in 1990. His government abolished term limits more than a decade ago. He has pledged to reintroduce them, but that has not happened. Results of the latest elections, held earlier this month, are expected to be released in the next several days.

Chad has one of the most able armies in the region, and Mr. Déby has played a central role in efforts backed by the United States to combat Boko Haram and other militants.

Gen. Lamidi Oyebayo Adeosun, the Nigerian commander of the regional group of African countries fighting Boko Haram in the Lake Chad area, said military officials were still trying to learn more about relations between Boko Haram and the Islamic State. He said Boko Haram had increased its attacks on soft targets like markets and mosques in recent months, and moved away from attacks on military bases.

General Adeosun showed reporters photos of what he said were the latest improvised explosive devices being used by Boko Haram, including a photo of a bird wrapped in explosives. Boko Haram, he said, is increasingly using children — especially girls — to deliver its explosive packages to markets. The group often drugged young girls, wrapped them in explosives and sent them into crowds, he said. “They are appealing to the natural care you feel for a girl child,” he said. “When you see a girl child, you will not feel that she could be carrying a bomb. They achieve maximum effect that way.”

The rising threat from the Islamic State in Libya comes as Mr. Obama is being asked by some of his top military and intelligence advisers to approve the broader use of American military force in Libya and Nigeria. Mr. Obama, administration officials said, is mulling how large a military campaign to order for Libya, and whether to approve sending additional Special Operations advisers and trainers to Nigeria.

The Pentagon has proposed spending $200 million this year to help train and equip the armies and security forces of North, Central and West African countries. The United States is also constructing a $50 million drone base in Agadez, Niger, that will allow Reaper surveillance aircraft to fly over the Lake Chad area.