The leader of a small Moslem sect was hanged publicly in Khartoum, the Sudanese capital, today for opposing the application of Islamic law, an event likely to stir renewed controversy over the rule of ailing President Jaafar Nimeri.

The hanging took place amid reports of a new upsurge in fighting between the Army and rebels in southern Sudan, with the government saying for the first time that unidentified aircraft had been used against its forces. If true, this would represent a significant new development in the conflict there.

The allegations are likely to be used by Nimeri to ask for more arms, including warplanes and helicopters, from Washington. An earlier request was turned down.

The public hanging of Mahmoud Mohammed Taha, 76, head of a banned group known as the Republican Brothers, was described here as unusual because of his age and the nature of the charges against him -- opposing Nimeri's decision to apply to the letter the sharia, or Islamic law.

The decision, made 16 months ago, has been opposed bitterly by the Christian minority in the south and publicly criticized by Egypt and the United States, Nimeri's two most important foreign backers.

The State Department last year termed the application of Islamic law to Christians in Sudan a violation of human rights and the practice of severing limbs of convicted criminals a cruel and unusual punishment.

In Washington, the State Department issued a statement Friday saying the United States "deplores the execution" of Taha and the sentencing of three of his followers "who have never been charged with crimes of violence but have been condemned for religious and political activities." It called the action "a clear violation of human rights."

Only last September, it appeared that Nimeri had gone back on his decision under pressure from Washington and Cairo. He disbanded the special courts he had set up to apply the sharia and halted the cutting off of hands and feet of those found guilty of theft and other crimes.

Taha, hanged before a chanting crowd of 2,000, had been sentenced with three colleagues by a criminal court earlier this month for "heresy and opposition to the way Moslem law is being implemented in the Sudan."

He was taken today, hooded and in chains, to the central prison in Khartoum and was hanged on a red-painted gallows standing in the main courtyard.

The Republican Brothers had also been involved in the spreading opposition to Nimeri's government, and Taha may have been singled out for execution because of this.

An appeals court had given the four condemned men a month to repent for their "heresy," but Nimeri stepped in yesterday to confirm the death sentence on Taha. He reduced the others' sentences to three days. It was not immediately clear what had led to his new hard-line attitude toward opponents of his decision to make the sharia the law of the land.

Nimeri's behavior over the past several years, blamed in part on the influence of several Moslem Sufi mystic advisers, has been a cause of increasing concern here in Cairo and in Washington.

At the same time, a rebellion against his rule by Christian-led southerners, backed by Libya and Ethiopia, has grown steadily worse, with major clashes taking place between the Army and guerrillas of the Sudanese People's Liberation Army during the past few weeks.

The rebels are seeking the overthrow of Nimeri rather than independence for the south, the main aim of another rebellion that dragged on for 17 years before ending in an agreement on local autonomy in 1973.

Today, the Sudan News Agency reported a new twist to the expanding insurgency against the Nimeri government. It said "two hostile aircraft" had intervened Wednesday and yesterday on behalf of the rebels during fighting near the Ugandan border.

The agency, quoting an Army spokesman, said 11 soldiers were killed and 25 wounded when the aircraft raided the headquarters of an Army contingent stationed in the town of Lafon near Torit, which is about 70 miles southeast of Juba, the capital of southern Sudan.

The spokesman was quoted as saying the hostile aircraft had fired "air-to-ground missiles" causing the destruction of some Army vehicles and inflicting an unannounced number of casualties.

"Two helicopters were also seen operating with the outlaws," the agency statement said. "It has been proven to our forces that a foreign party, which will be identified subsequently, is providing air support to the outlaws," it added.

The Sudanese government has accused Libya and Ethiopia of providing arms, training, artillery and logistical support to the southern rebel army, which is led by John Garang, a U.S.-educated former Army officer.

The implication of the latest Sudanese accusation is that one of the two, presumably using Ethiopian bases, has decided to escalate its aid by providing air support for the rebels.